{"id":1811,"date":"2020-02-16T16:08:17","date_gmt":"2020-02-16T16:08:17","guid":{"rendered":"https:\/\/theduff.co.uk\/?p=1811"},"modified":"2020-12-10T09:53:09","modified_gmt":"2020-12-10T09:53:09","slug":"chromosomal-abnormalities-aneuploidy-ivf","status":"publish","type":"post","link":"https:\/\/theduff.co.uk\/chromosomal-abnormalities-aneuploidy-ivf\/","title":{"rendered":"<strong>Chromosomal abnormalities<\/strong> and IVF"},"content":{"rendered":"\n<p><strong>IF YOU'RE HERE BECAUSE <\/strong>you\u2019re revising for your school biology exams but didn\u2019t understand your teacher\u2019s explanations\u2026 this page isn\u2019t <em>quite <\/em>intended for you, but welcome anyway! I hope you find it useful.&nbsp;<\/p>\n\n\n\n<p>If you\u2019re here because you\u2019re trying to get pregnant, you\u2019re definitely in the right place. Once you\u2019ve finished reading, you\u2019ll have a far better understanding of your embryos, how chromosomal defects occur, who\u2019s at risk, and lots more.<\/p>\n\n\n<style>.elementor-572 .elementor-element.elementor-element-189190b1{padding:0% 36% 8% 22%;}.elementor-572 .elementor-element.elementor-element-6b407887 > .elementor-element-populated{margin:0% 3.2% 0% 14%;--e-column-margin-right:3.2%;--e-column-margin-left:14%;}.elementor-572 .elementor-element.elementor-element-65191d58{--box-background-color:#FCFAEE;--box-border-color:#007595;--box-border-width:0px;--box-border-radius:0px;--box-padding:24px;--box-min-height:48vh;--header-color:#F16653;--toggle-button-color:#333333;--item-text-color:#007595;--item-text-decoration:underline;--item-text-hover-decoration:underline;--marker-color:#007595;}.elementor-572 .elementor-element.elementor-element-65191d58 .elementor-toc__header, .elementor-572 .elementor-element.elementor-element-65191d58 .elementor-toc__header-title{font-size:20px;}.elementor-572 .elementor-element.elementor-element-65191d58 .elementor-toc__list-item{font-size:20px;}.elementor-572 .elementor-element.elementor-element-65191d58 > .elementor-widget-container{margin:0px 0px 0px 4px;}@media(max-width:1024px){.elementor-572 .elementor-element.elementor-element-189190b1{padding:0% 4% 10% 14%;}.elementor-572 .elementor-element.elementor-element-6b407887 > .elementor-element-populated{margin:0% 10% 0% 0%;--e-column-margin-right:10%;--e-column-margin-left:0%;padding:0px 0px 0px 0px;}}@media(max-width:767px){.elementor-572 .elementor-element.elementor-element-189190b1{margin-top:16%;margin-bottom:18%;padding:0% 0% 0% 0%;}.elementor-572 .elementor-element.elementor-element-6b407887 > .elementor-element-populated{margin:0% 0% 0% 0%;--e-column-margin-right:0%;--e-column-margin-left:0%;padding:0% 0% 0% 0%;}.elementor-572 .elementor-element.elementor-element-65191d58 > .elementor-widget-container{margin:0px 0px 0px 0px;padding:0px 0px 0px 0px;}}\/* Start custom CSS for table-of-contents, class: .elementor-element-65191d58 *\/.elementor-toc__header-title {\n    color: #333333;\n}\n\n.elementor-toc__list-wrapper {\n    margin: 0em 2em 2em 0em;\n}\n\n.elementor-toc__list-items--collapsible li:not(:last-of-type) {\n    margin: 1.5em 0em;\n    padding-bottom: 1.5em;\n    border-bottom: 0.5px solid #007595;\n}\n\n\/* --- For Mobile --- *\/\n@media only screen and (max-width: 400px) {\n    .elementor-toc__list-wrapper {\n    margin: -1em 0em 0em 0em;\n}\n\n.elementor-toc__list-items--collapsible li:not(:last-of-type) {\n    margin: 1.2em 0em;\n    padding-bottom: 1.2em;\n    border-bottom: 0.5px solid #007595;\n}\n}\/* End custom CSS *\/<\/style>\t\t<div data-elementor-type=\"section\" data-elementor-id=\"572\" class=\"elementor elementor-572\" data-elementor-post-type=\"elementor_library\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-top-section elementor-element elementor-element-189190b1 elementor-section-stretched elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"189190b1\" data-element_type=\"section\" data-settings=\"{&quot;stretch_section&quot;:&quot;section-stretched&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-6b407887\" data-id=\"6b407887\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-65191d58 elementor-toc--minimized-on-none elementor-widget elementor-widget-table-of-contents\" data-id=\"65191d58\" data-element_type=\"widget\" data-settings=\"{&quot;exclude_headings_by_selector&quot;:[],&quot;collapse_subitems&quot;:&quot;yes&quot;,&quot;min_height&quot;:{&quot;unit&quot;:&quot;vh&quot;,&quot;size&quot;:48,&quot;sizes&quot;:[]},&quot;headings_by_tags&quot;:[&quot;h2&quot;],&quot;minimized_on&quot;:&quot;none&quot;,&quot;marker_view&quot;:&quot;bullets&quot;,&quot;icon&quot;:{&quot;value&quot;:&quot;fas fa-circle&quot;,&quot;library&quot;:&quot;fa-solid&quot;},&quot;minimize_box&quot;:&quot;yes&quot;,&quot;hierarchical_view&quot;:&quot;yes&quot;,&quot;min_height_tablet&quot;:{&quot;unit&quot;:&quot;px&quot;,&quot;size&quot;:&quot;&quot;,&quot;sizes&quot;:[]},&quot;min_height_mobile&quot;:{&quot;unit&quot;:&quot;px&quot;,&quot;size&quot;:&quot;&quot;,&quot;sizes&quot;:[]}}\" data-widget_type=\"table-of-contents.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<div class=\"elementor-toc__header\">\n\t\t\t<h4 class=\"elementor-toc__header-title\">\n\t\t\t\tContents\t\t\t<\/h4>\n\t\t\t\t\t\t\t<div class=\"elementor-toc__toggle-button elementor-toc__toggle-button--expand\" role=\"button\" tabindex=\"0\" aria-controls=\"elementor-toc__65191d58\" aria-expanded=\"true\" aria-label=\"Open table of contents\"><i aria-hidden=\"true\" class=\"fas fa-chevron-down\"><\/i><\/div>\n\t\t\t\t<div class=\"elementor-toc__toggle-button elementor-toc__toggle-button--collapse\" role=\"button\" tabindex=\"0\" aria-controls=\"elementor-toc__65191d58\" aria-expanded=\"true\" aria-label=\"Close table of contents\"><i aria-hidden=\"true\" class=\"fas fa-chevron-up\"><\/i><\/div>\n\t\t\t\t\t<\/div>\n\t\t<div id=\"elementor-toc__65191d58\" class=\"elementor-toc__body elementor-toc__list-items--collapsible\">\n\t\t\t<div class=\"elementor-toc__spinner-container\">\n\t\t\t\t<i class=\"elementor-toc__spinner eicon-animation-spin eicon-loading\" aria-hidden=\"true\"><\/i>\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t<\/div>\n\t\t\n\n\n<h2 class=\"wp-block-heading\"><strong>Why the fuss about chromosomes?<\/strong><\/h2>\n\n\n\n<p>Chromosomal issues are one of the leading causes of implantation failure, miscarriage and birth defects among babies. Some people are more at risk of producing embryos with chromosomal issues than others.&nbsp;<\/p>\n\n\n\n<p>There are two types of screening that intend to discover if any chromosomal issues exist in your embryos: <a href=\"\/pgs-testing-ivf\/\">PGS<\/a> (preimplantation genetic screening) and PGD (preimplantation genetic diagnosis). This article isn't about PGS or PGD, though: instead, it explains why and how chromosomal issues appear in the first place. <\/p>\n\n\n\n<p>Think of this article as containing all the background information you need to fully understand PGS and PGD (rather than \u201cessential reading\u201d).&nbsp;<\/p>\n\n\n\n<ul><li>If you\u2019d rather just learn about the PGS process, how it works, the success rates, pros and cons, etc., <a href=\"\/pgs-testing-ivf\/\">read this article about PGS instead<\/a>.&nbsp;<\/li><li>I haven\u2019t yet written an article about PGD, but it\u2019s on the to-do list!&nbsp;<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>What is a chromosome?<\/strong><\/h2>\n\n\n\n<p>A chromosome is a small, dense strand of DNA, which contains genes that tell your body how to develop and function. (Chromosomes also contain proteins, which will become relevant later.)<br><\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img fetchpriority=\"high\" decoding=\"async\" width=\"400\" height=\"188\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/one-chromosome.jpeg\" alt=\"One chromosome\" class=\"wp-image-1793\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/one-chromosome.jpeg 400w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/one-chromosome-300x141.jpeg 300w\" sizes=\"(max-width: 400px) 100vw, 400px\" \/><\/figure><\/div>\n\n\n\n<p>Every cell in the human body should contain 23 pairs of chromosomes \u2013 making 46 chromosomes in total. A set of 46 chromosomes, as seen under a microscope, is known as a \u201ckaryotype\u201d. Like this:&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img decoding=\"async\" width=\"1020\" height=\"1024\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/normal-chromosome-1020x1024.png\" alt=\"Karyotype of normal chromosome\" class=\"wp-image-1792\"\/><figcaption>With thanks to Wessex Reg. Genetics Centre, Wellcome Images<\/figcaption><\/figure><\/div>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Some background info on these 23 pairs of chromosomes\u2026&nbsp;<\/strong><\/h3>\n\n\n\n<p>So\u2026 every cell in the human body should contain 23 pairs of chromosomes \u2013 making 46 chromosomes in total.&nbsp;<\/p>\n\n\n\n<p>Every cell <em>except<\/em> egg and sperm cells, which contain just one half of the set each. That is, the egg contains 23 chromosomes and the sperm contains 23 chromosomes. <\/p>\n\n\n\n<p>When an egg and sperm fuse during fertilisation, the chromosomes from the egg and sperm pair up into matched sets known as \u201chomologous\u201d pairs (with the slight exception of the final, 23rd pair, which we\u2019ll come to shortly).<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img decoding=\"async\" width=\"366\" height=\"261\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/23-chromosomes-each.jpg\" alt=\"23 chromosomes from the egg, and 23 chromosomes from the sperm\" class=\"wp-image-1774\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/23-chromosomes-each.jpg 366w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/23-chromosomes-each-300x214.jpg 300w\" sizes=\"(max-width: 366px) 100vw, 366px\" \/><figcaption>With thanks to https:\/\/www.womenfitness.net\/<\/figcaption><\/figure><\/div>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>The first 22 pairs of chromosomes are called \u201cautosomes\u201d<\/strong><\/h4>\n\n\n\n<p>These autosomes are numbered from 1 to 22 (from largest to smallest). The autosomes contain most of your genetic information, such as height, eye colour, and so on.&nbsp;<\/p>\n\n\n\n<p>\u201cHomologous\u201d means that the chromosomes in each pair share the same size and shape, and carry the same type of genetic information \u2013 but it doesn\u2019t<em> <\/em>mean they\u2019re identical. Even though homologous chromosomes carry the same type of genetic information, they don\u2019t necessarily have the same <em>versions <\/em>of genes. That\u2019s because one of those chromosomes came from the mother, while the other came from the father.&nbsp;<\/p>\n\n\n\n<p>Here\u2019s an example to show what I mean:&nbsp;<\/p>\n\n\n\n<p>There\u2019s a gene on Chromosome pair #9 that determines blood type. It\u2019s possible for a person to have two identical copies of the gene \u2013 one on each homologous chromosome. E.g. a person may have the \u201cType A blood\u201d gene from both the mother and father. But a person <em>could <\/em>have two different gene versions \u2013 such as one for \u201cType A blood\u201d and one for \u201cType B blood\u201d.&nbsp;<\/p>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>The final, 23rd pair is called the \u201csex chromosomes\u201d<\/strong><\/h4>\n\n\n\n<p>And it determines a person\u2019s shoe size.&nbsp;<\/p>\n\n\n\n<p>Kidding! The sex chromosomes determine a person\u2019s sex.&nbsp;<\/p>\n\n\n\n<p>The 23rd chromosome of the egg is always referred to as an \u201cX\u201d chromosome, but the 23rd chromosome of the sperm can be an \u201cX\u201d or a \u201cY\u201d. When the egg and sperm fuse during fertilisation, the resulting 23rd pair can either be \u201cXX\u201d or \u201cXY\u201d, depending on which chromosome the sperm is carrying. An \u201cXX\u201d pair indicates that the embryo is female, and an \u201cXY\u201d pair indicates that it\u2019s male.&nbsp;<\/p>\n\n\n\n<p>If you squint a bit and blink a bit and hold one eye shut, the \u201cX\u201d sort of looks like an X and the \u201cY\u201d sort of looks like a Y. You might need a few glasses of wine to really see it: <\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"450\" height=\"252\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/x-and-y.png\" alt=\"Difference between male and female cells: the x and y chromosomes\" class=\"wp-image-1803\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/x-and-y.png 450w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/x-and-y-300x168.png 300w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><figcaption>With thanks to https:\/\/genetics.thetech.org\/<\/figcaption><\/figure><\/div>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>All about chromosomal abnormalities&nbsp;<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What is a chromosomal abnormality?<\/strong><\/h3>\n\n\n\n<p>Any deviation from the normal karyotype is known as a chromosomal abnormality; at least <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC1683793\/\">50%<\/a> of all miscarriages are due to a chromosomal abnormality. There are two main types of deviation:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Numerical abnormalities<\/strong> \u2013 where the number of chromosomes in each cell is anything other than 46 (or 23 pairs). Numerical abnormalities are far more prevalent than structural ones, and they account for about <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC1683793\/\">95%<\/a> of the miscarriages that are caused by chromosomal abnormalities. Only about <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pubmed\/7789361\">0.3%<\/a> of babies are born with a numerical abnormality, because most are miscarried. <br><br>When embryos have the incorrect number of chromosomes, they\u2019re called \u201caneuploid\u201d. When they have the correct number, they\u2019re called \u201ceuploid\u201d.<\/li><li><strong>Structural abnormalities<\/strong> \u2013 where large sections of DNA are either missing from or added to a chromosome. They account for the remaining <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC1683793\/\">5%<\/a> of miscarriages that are caused by chromosomal abnormalities. (But unlike numerical abnormalities, it\u2019s far more likely that babies can survive with structural abnormalities \u2013 and some are completely harmless.)&nbsp;<\/li><\/ul>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>How common are chromosomal abnormalities in embryos?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Frighteningly common. According to <a href=\"https:\/\/www.fertstert.org\/article\/S0015-0282(18)30281-4\/fulltext#tbl2\">one study<\/a>, 42.9% of embryos (across all age ranges) are found to be aneuploid.&nbsp;<\/p>\n\n\n\n<p>But chromosomal abnormalities increase dramatically with maternal age. Here\u2019s some data from <a href=\"https:\/\/academic.oup.com\/humrep\/article\/27\/10\/3109\/748419\">a study conducted in 2012<\/a> (note that the second column shows the percentage of <strong>normal <\/strong>embryos):&nbsp;<\/p>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table><tbody><tr><td><strong>Maternal age<\/strong><\/td><td><strong>% normal embryos<\/strong><\/td><\/tr><tr><td>&lt; 25 years<\/td><td>70%<\/td><\/tr><tr><td>25\u201329 years<\/td><td>48%<\/td><\/tr><tr><td>30\u201334 years<\/td><td>42%<\/td><\/tr><tr><td>35\u201339 years<\/td><td>34%<\/td><\/tr><tr><td>&gt; 39 years<\/td><td>11%<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Igenomix (a reproductive genetics company) had similar results:&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"484\" height=\"135\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/Igenomix.jpg\" alt=\"Numerical abnormalities in embryos, by age\" class=\"wp-image-1784\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/Igenomix.jpg 484w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/Igenomix-300x84.jpg 300w\" sizes=\"(max-width: 484px) 100vw, 484px\" \/><figcaption>With thanks to Igenomix<\/figcaption><\/figure><\/div>\n\n\n\n<p>And <a href=\"https:\/\/www.fertstert.org\/article\/S0015-0282(13)03257-3\/abstract\">this study<\/a> from 2014 found that women under the age of 35 had a 60% chance of a blastocyst (the name for an embryo <a href=\"https:\/\/theduff.co.uk\/zygote-embryo-blastocyst-foetus-ivf\/\">five or six days after fertilisatio<\/a>n) being euploid, which reduced to 30% by the age of 41. The chance of getting NO euploids was about 10% for under 35s and about 50% by the age of 43. Other studies (like <a href=\"https:\/\/www.fertstert.org\/article\/S0015-0282(13)02811-2\/abstract\">this one<\/a>, but many others too) have the same sorts of figures.&nbsp;<\/p>\n\n\n\n<p>Why do abnormalities become more common as women get older? We\u2019ll cover that in a LOT of detail later!&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>If one cell in a human body contains chromosomal abnormalities, does that mean <\/strong><strong><em>all <\/em><\/strong><strong>cells in that body contain chromosomal abnormalities?<\/strong><\/h3>\n\n\n\n<p>For the purposes of the discussion right now, we\u2019ll say \u201cyes, usually\u201d. But if you want the full, nuanced answer, read my article about <a href=\"\/pgs-testing-ivf\/\">PGS testing<\/a> (read the bit about &#8220;mosaicism&#8221;).&nbsp;<\/p>\n\n\n\n<p>Here\u2019s why:&nbsp;<\/p>\n\n\n\n<ul><li>When a sperm fertilises an egg, a single cell called a zygote is formed. The zygote contains all 46 chromosomes (23 from the egg and 23 from the sperm).<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"357\" height=\"288\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Zygote-to-Foetus-02-700px-1-e1584284057630.png\" alt=\"Zygote\" class=\"wp-image-793\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Zygote-to-Foetus-02-700px-1-e1584284057630.png 357w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Zygote-to-Foetus-02-700px-1-e1584284057630-300x242.png 300w\" sizes=\"(max-width: 357px) 100vw, 357px\" \/><\/figure><\/div>\n\n\n\n<ul><li>The new cell divides in two for the first time after 24 hours and it\u2019s no longer called a zygote: it\u2019s now an embryo. A short while later, it divides in two yet again. (If you want to do more Googling about this process later, it\u2019s called \u201cmitosis\u201d.)<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"294\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Embryos.png\" alt=\"Two-cell embryo and four-cell embryo\" class=\"wp-image-775\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Embryos.png 500w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Embryos-300x176.png 300w\" sizes=\"(max-width: 500px) 100vw, 500px\" \/><\/figure><\/div>\n\n\n\n<ul><li>This cell division keeps happening until you end up with so many cells that they can\u2019t be counted. <\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"488\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/morula-\u2013-how-many.jpg\" alt=\"Morula: lots of cells\" class=\"wp-image-1817\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/morula-\u2013-how-many.jpg 500w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/morula-\u2013-how-many-300x293.jpg 300w\" sizes=\"(max-width: 500px) 100vw, 500px\" \/><\/figure><\/div>\n\n\n\n<ul><li>And now for the important bit: as the embryo grows and the cells divide, the chromosomes are copied in every single cell. All the cells in a human body are made this way, which means every cell has an identical set of chromosomes. <strong>An error in the first cell, therefore, will be replicated in every other cell.&nbsp;<\/strong><\/li><\/ul>\n\n\n\n<p>There\u2019s a caveat to all this, which we\u2019ll come to later. But for now, this is all you need to know.&nbsp;<\/p>\n\n\n\n<p>Now let\u2019s look at each of the different chromosomal abnormalities in turn.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>About numerical abnormalities (AKA \u201cchromosomal aneuploidy\u201d)<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What are \u201caneuploid\u201d chromosomes all about?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Chromosomal aneuploidy (try writing that while drunk) is when there\u2019s any number other than 46 chromosomes in a cell. A cell with 45 chromosomes is aneuploid\/abnormal \u2013 as is a cell with 47 or 48. And so on. Remember that if one cell in the human body is aneuploid, the other cells in the human body will also be aneuploid.&nbsp;<\/p>\n\n\n\n<p>There are two common types of aneuploidy:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Monosomy<\/strong>: when a chromosome in a particular pair is missing (giving a total of 45 chromosomes instead of 46)<\/li><li><strong>Trisomy<\/strong>: when there\u2019s an extra copy of a chromosome \u2013 that is, there are three chromosomes instead of two (giving a total of 47 chromosomes instead of 46)<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"872\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/complete-genome-1024x872.jpg\" alt=\"Examples of trisomy and monosomy\" class=\"wp-image-1820\"\/><figcaption>With thanks to https:\/\/learn.genetics.utah.edu\/content\/disorders\/extraormissing\/ <\/figcaption><\/figure><\/div>\n\n\n\n<p>There are two other types, but they\u2019re far less common \u2013 so they won\u2019t get discussed much in this article:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Nullisomy<\/strong>: when both chromosomes in a particular pair are missing (giving a total of 44 chromosomes instead of 46)<\/li><li><strong>Tetrasomy<\/strong>: when there are a total of four chromosomes rather than two in a pair (giving a total of 48 chromosomes instead of 46)<\/li><\/ul>\n\n\n\n<p><strong>Whatever the abnormality, what you need to know is this: when there are too many or too few copies of a chromosome, the cell gets into all sorts of bother and confusion.&nbsp;<\/strong><\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What are the risks associated with numerical abnormalities?&nbsp;<\/strong><\/h3>\n\n\n\n<p>As mentioned above, aneuploidy is the leading cause of implantation failure and miscarriage. Some embryos do survive to term, but the baby will either die within a few days of birth or have a <a href=\"https:\/\/www.intechopen.com\/online-first\/aneuploidy-rates-inversely-correlate-with-implantation-during-in-vitro-fertilization-procedures-in-f\">major developmental disability<\/a>. When the abnormality is restricted to the sex chromosomes, the outlook is generally more positive.&nbsp;<\/p>\n\n\n\n<p>Here are the most common numerical abnormalities that can survive pregnancy:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Down syndrome (\u201ctrisomy 21\u201d) \u2013 <\/strong>when<strong> <\/strong>there\u2019s an extra chromosome in pair 21. According to one paper in the science journal <a href=\"https:\/\/www.nature.com\/scitable\/nated\/article?action=showContentInPopup&contentPK=290\">Nature<\/a>, \u201cIndividuals with DS all demonstrate signs of cognitive impairment, along with a more variable range of other symptoms, but patients routinely live to adulthood.\u201d <\/li><li><strong>Patau syndrome (\u201ctrisomy 13\u201d)<\/strong>. According to the <a href=\"https:\/\/www.nhs.uk\/conditions\/pataus-syndrome\/\">NHS website<\/a>, trisomy 13, \u201cin many cases, results in miscarriage, stillbirth or the baby dying shortly after birth\u201d. It goes on to say that \u201cMore than 9 out of 10 children born with [Patau syndrome] die during the first year\u201d. <\/li><li><strong>Edward syndrome (\u201ctrisomy 18\u201d)<\/strong>. The NHS says, \u201cSadly, most babies with [Edward syndrome] will die before or shortly after being born.\u201d<\/li><li>Embryos with extra sex chromosomes, such as <strong>Klinefelter syndrome \u2013 <\/strong>where males have two X chromsomes plus one Y chromosome or sometimes one X and two Ys (in addition to the 44 autosomes \u2013 making 47 chromosomes in total). Females have three Xs. Compared to most other numerical abnormalities, \u201c&#8230; these sorts of sex chromosome trisomies are fairly benign. Affected individuals generally show reduced sexual development and fertility, but they often have normal life spans, and many of their symptoms can be treated by hormone supplementation.\u201d (<a href=\"https:\/\/www.nature.com\/scitable\/topicpage\/chromosomal-abnormalities-aneuploidies-290\/\">Source<\/a>.)<\/li><li>Females with a single copy of the X chromosome have a condition called <strong>Turner syndrome<\/strong>; their symptoms include growing up to be shorter than average, having underdeveloped ovaries, and certain visual characteristics. But while there\u2019s no cure, many of the symptoms can be treated \u2013 and people with Turner syndrome can lead fairly normal lives. <\/li><li><strong>Monosomy 21<\/strong>. Babies rarely survive longer than a few weeks if the chromosome is completely missing, but they can survive is there\u2019s only a \u201cpartial deletion\u201d of a chromosome. Babies who survive (due to only a partial deletion) have symptoms similar to those of Down syndrome.&nbsp;<\/li><\/ul>\n\n\n\n<p><strong>But remember: most numerical abnormalities don\u2019t even survive pregnancy.&nbsp;&nbsp;<\/strong><\/p>\n\n\n\n<p>Here are some karyotype images showing different numerical chromosomal abnormalities:<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"989\" height=\"1024\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/different-numerical-abnormalities-989x1024.jpg\" alt=\"Karyotypes with different numerical abnormalities\" class=\"wp-image-1781\"\/><figcaption>With thanks to Wessex Reg. Genetics Centre, Wellcome Images<\/figcaption><\/figure><\/div>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What causes aneuploid embryos?&nbsp;<\/strong><\/h3>\n\n\n\n<p>There are two main ways in which embryo aneuploidy can happen:&nbsp;<\/p>\n\n\n\n<p>Either\u2026&nbsp;<\/p>\n\n\n\n<ul><li>Soon after fertilisation, something can go wrong during cell division (mitosis) and the chromosomes won\u2019t be distributed evenly. This cause doesn\u2019t seem linked to maternal age.&nbsp;<\/li><\/ul>\n\n\n\n<p>Or (more likely \u2013 especially in older women)&#8230;<\/p>\n\n\n\n<ul><li>The egg cell or sperm cell (but usually the egg cell) could contain an abnormal number of chromosomes. If this egg or sperm gets fertilised, the resulting embryo will have too many or too few chromosomes. <br><br>This particular problem relates to a process called meiosis, which will be covered in a lot of detail shortly.&nbsp;<\/li><\/ul>\n\n\n\n<p>Is aneuploidy ever an inherited condition? No. BUT structural abnormalities <em>can<\/em> be inherited, and the structural changes can lead to numerical changes. This will be covered later.&nbsp;<\/p>\n\n\n\n<p>It\u2019s important to remember that most of the time, the \u201coriginal cause\u201d is just bad luck. Yes: older women are more likely to have numerically abnormal eggs, but younger women have them too. And whether the original cause is an egg with the wrong number of chromosomes or an error in mitosis, it\u2019s often just a case of Sod\u2019s Law.&nbsp;<\/p>\n\n\n\n<p>So remember: a healthy young woman with no family history of chromosomal abnormalities may still become pregnant with an abnormal embryo.&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large is-resized\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x.png\" alt=\"\" class=\"wp-image-492\" width=\"229\" height=\"105\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x.png 443w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x-300x137.png 300w\" sizes=\"(max-width: 229px) 100vw, 229px\" \/><\/figure><\/div>\n\n\n\n<p><\/p>\n\n\n<style>.elementor-1039 .elementor-element.elementor-element-6e3b2cc{margin-top:2%;margin-bottom:12%;padding:1% 4% 1% 4%;z-index:2;}.elementor-1039 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.elementor-widget-container{margin:6% 0% 2% 0%;padding:0px 0px 0px 0px;}.elementor-1039 .elementor-element.elementor-element-b952531{font-size:1em;}.elementor-1039 .elementor-element.elementor-element-df03e60 .elementor-field-group .elementor-field, .elementor-1039 .elementor-element.elementor-element-df03e60 .elementor-field-subgroup label{font-size:1em;}.elementor-1039 .elementor-element.elementor-element-df03e60 .elementor-button{font-size:0.8em;letter-spacing:0.2px;border-width:0px 60px 0px 60px;}.elementor-1039 .elementor-element.elementor-element-df03e60 > .elementor-widget-container{margin:-4% 0% 10% 0%;padding:0px 0px 0px 0px;}.elementor-1039 .elementor-element.elementor-element-861ee7c{font-size:1em;}}\/* Start custom CSS for button, class: .elementor-element-53798fd *\/#fdg {\n    color: #FFFFFF;\n}\n#fdg:hover {\n    color: #FFFFFF;\n}\/* End custom CSS *\/<\/style>\t\t<div data-elementor-type=\"section\" data-elementor-id=\"1039\" class=\"elementor elementor-1039\" data-elementor-post-type=\"elementor_library\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-top-section elementor-element elementor-element-6e3b2cc elementor-section-stretched elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"6e3b2cc\" data-element_type=\"section\" data-settings=\"{&quot;stretch_section&quot;:&quot;section-stretched&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-cab4a86\" data-id=\"cab4a86\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-inner-section elementor-element elementor-element-4a9f61d elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"4a9f61d\" data-element_type=\"section\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-inner-column elementor-element elementor-element-f69664e\" data-id=\"f69664e\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-53798fd elementor-align-center elementor-widget elementor-widget-button\" data-id=\"53798fd\" data-element_type=\"widget\" data-widget_type=\"button.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<div class=\"elementor-button-wrapper\">\n\t\t\t<a class=\"elementor-button elementor-size-sm\" role=\"button\" id=\"fdg\">\n\t\t\t\t\t\t<span class=\"elementor-button-content-wrapper\">\n\t\t\t\t\t\t<span class=\"elementor-button-text\">FREE EMAIL<\/span>\n\t\t<\/span>\n\t\t\t\t\t<\/a>\n\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-6567c58 elementor-widget elementor-widget-heading\" data-id=\"6567c58\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<p class=\"elementor-heading-title elementor-size-default\">One embryo or two?<\/p>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b952531 elementor-widget elementor-widget-text-editor\" data-id=\"b952531\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p>What are the risks and benefits of transferring more than one embryo? When clinics and hospitals recommend a single embryo transfer, do they have an underlying incentive?<\/p><p>What are the real facts?<\/p><p>I dug deep into all the research so you don't have to! Enter your email below for an easy-to-read, easy-to-understand guide to &#8220;one embryo vs two&#8221;.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-df03e60 elementor-mobile-button-align-center elementor-tablet-button-align-stretch elementor-button-align-stretch elementor-widget elementor-widget-form\" data-id=\"df03e60\" data-element_type=\"widget\" data-settings=\"{&quot;button_width&quot;:&quot;20&quot;,&quot;button_width_mobile&quot;:&quot;100&quot;,&quot;button_width_tablet&quot;:&quot;25&quot;,&quot;step_next_label&quot;:&quot;Next&quot;,&quot;step_previous_label&quot;:&quot;Previous&quot;,&quot;step_type&quot;:&quot;number_text&quot;,&quot;step_icon_shape&quot;:&quot;circle&quot;}\" data-widget_type=\"form.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<form class=\"elementor-form\" method=\"post\" name=\"New Form\">\n\t\t\t<input type=\"hidden\" name=\"post_id\" value=\"1039\"\/>\n\t\t\t<input type=\"hidden\" name=\"form_id\" value=\"df03e60\"\/>\n\t\t\t<input type=\"hidden\" name=\"referer_title\" value=\"The Duff\" \/>\n\n\t\t\t\n\t\t\t<div class=\"elementor-form-fields-wrapper elementor-labels-\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-field-type-email elementor-field-group elementor-column elementor-field-group-email elementor-col-80 elementor-md-75 elementor-sm-100 elementor-field-required\">\n\t\t\t\t\t\t\t\t\t\t\t\t<label for=\"form-field-email\" class=\"elementor-field-label elementor-screen-only\">\n\t\t\t\t\t\t\t\tEmail\t\t\t\t\t\t\t<\/label>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t<input size=\"1\" type=\"email\" name=\"form_fields[email]\" id=\"form-field-email\" class=\"elementor-field elementor-size-lg  elementor-field-textual\" placeholder=\"Your email address\" required=\"required\" aria-required=\"true\">\n\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t<div class=\"elementor-field-group elementor-column elementor-field-type-submit elementor-col-20 e-form__buttons elementor-md-25 elementor-sm-100\">\n\t\t\t\t\t<button type=\"submit\" class=\"elementor-button elementor-size-lg\">\n\t\t\t\t\t\t<span >\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<span class=\" elementor-button-icon\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-button-text\">Get the facts<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t<\/button>\n\t\t\t\t<\/div>\n\t\t\t<\/div>\n\t\t<\/form>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-861ee7c elementor-widget elementor-widget-text-editor\" data-id=\"861ee7c\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><strong>I won't send you spam. Unsubscribe at any time.<\/strong><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t<\/div>\n\t\t\n\n\n<h3 class=\"wp-block-heading\"><strong>Why are older women more likely to have egg cells with the incorrect number of chromosomes?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Before I attempt to answer this, it\u2019s worth giving you some background information about how egg cells are formed in the first place.&nbsp;<\/p>\n\n\n\n<p>So what you\u2019ll now experience is a massive tangent in which I attempt to explain something called meiosis. Apparently, I learnt all this in GCSE biology many years ago. I have absolutely no recollection of that experience (something I\u2019ll blame on the distracting cuteness of my teacher\u2019s arse), so I\u2019ve just spent about a fortnight\u2019s worth of spare time trying to figure it out from scratch.&nbsp;<\/p>\n\n\n\n<p>You don\u2019t <em>need <\/em>to know how meiosis works, but it helps if you\u2019re the kind of person who likes to understand <em>everything <\/em>about a topic. Meiosis isn\u2019t a typo of \u201cmitosis\u201d, but it\u2019s a similar(ish) process: both feature division and duplication, but in different ways from each other.&nbsp;<\/p>\n\n\n\n<hr class=\"wp-block-separator has-text-color has-background has-very-dark-gray-background-color has-very-dark-gray-color is-style-wide\"\/>\n\n\n\n<div style=\"height:50px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Tangent begins here<\/strong><\/h3>\n\n\n\n<p>So, meiosis. Actually, let\u2019s start (briefly) with mitosis. Mitosis, if you\u2019ll remember, is when a cell continues to divide and duplicate. Mitosis occurs after an egg and sperm have fused together to form a zygote (a single cell with all 46 chromosomes). Mitosis works to replicate this single cell and its genetic material throughout the body.&nbsp;<\/p>\n\n\n\n<p>Whereas mitosis occurs throughout the human body, meiosis is used for just one purpose: to produce sex cells (AKA eggs and sperm, or &#8220;gametes&#8221;). <\/p>\n\n\n\n<p>As you should bloody well know by now, sex cells have half the usual number of chromosomes of a regular cell (that is, 23 instead of 46). It\u2019s so that the egg and sperm can fuse together during fertilisation and plonk their respective 23 chromosomes together (in \u201chomologous pairs\u201d) \u2013 resulting in a full-set zygote of 46 chromosomes.&nbsp;<\/p>\n\n\n\n<p>Meiosis is the process that makes it possible for the egg and sperm cells to have half the usual number of chromosomes. Without meiosis, egg cells and sperm cells would each contain 46 chromosomes. Without meiosis, therefore, humanity would have gone skewiff a LONG time ago.&nbsp;<\/p>\n\n\n\n<p>But how does meiosis do this?&nbsp;<\/p>\n\n\n\n<p>Unfortunately for all of us, the answer isn\u2019t \u201cmagic\u201d. The answer is, \u201cIt\u2019s a process. And it involves two main stages. Actually, three.\u201d (It\u2019s all relevant\u2026 promise.)<\/p>\n\n\n\n<p>Pull up a chair.&nbsp;<\/p>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>The interphase stage<\/strong><\/h4>\n\n\n\n<p>\u201cInterphase\u201d happens <em>before <\/em>meiosis, and it happens in the reproductive organs. It\u2019s when the chromosomes in a cell are replicated<em> <\/em>to create double the number of chromosomes \u2013 that is, 92 chromosomes in total. (Except it\u2019s not really 92 chromosomes \u2013 and we\u2019ll come to that shortly.) <\/p>\n\n\n\n<p>The replicated chromosomes are connected to each other by something called a \u201c<strong>centromere<\/strong>\u201d, and these replicated chromosomes sit in their homologous pairs.<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"302\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/replication-e1587226671588.jpg\" alt=\"The interphase stage of meiosis\" class=\"wp-image-1799\"\/><\/figure><\/div>\n\n\n\n<p><em>About the \u201cchromatids\u201d in the diagram: a chromatid is simply the name given to an individual chromosome that\u2019s been newly copied.&nbsp;<\/em><\/p>\n\n\n\n<p>In a \u201cregular\u201d, non-interphase-stage cell, you\u2019d have 23 homologous pairs of unreplicated chromosomes. By now you\u2019re familiar with this image:&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"391\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/regular-cell-with-23-pairs-e1587226721431.png\" alt=\"Karyotype of normal chromosome\" class=\"wp-image-1797\"\/><\/figure><\/div>\n\n\n\n<p>In an interphase-stage cell, you\u2019d have 23 pairs of replicated chromosomes. And it would look something like the image below. It\u2019s the best image I could find, but it\u2019s not great; hopefully you\u2019ll get the gist.&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"375\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/replicated-chromosomes-after-s-phase-of-interphase-l-e1587226784200.jpg\" alt=\"Karyotype of an interphase-stage cell\" class=\"wp-image-1798\"\/><figcaption>With thanks to https:\/\/www.slideserve.com\/clara\/intro-to-meiosis<\/figcaption><\/figure><\/div>\n\n\n\n<p>So\u2026 there are now 92 chromosomes in a single cell. Except we don\u2019t <em>say <\/em>there are 92 chromosomes because we count chromosomes according to how many centromeres there are.&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"459\" height=\"479\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/sister-chromatids-1423BB72DD74AF28537.jpg\" alt=\"Image of a duplicated chromosome\" class=\"wp-image-1801\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/sister-chromatids-1423BB72DD74AF28537.jpg 459w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/sister-chromatids-1423BB72DD74AF28537-287x300.jpg 287w\" sizes=\"(max-width: 459px) 100vw, 459px\" \/><\/figure><\/div>\n\n\n\n<p>If you happen to be fussed about details, we now have 92 chromatids but only 46 centromeres \u2013 and the experts say that means we have 46 chromosomes.&nbsp;<\/p>\n\n\n\n<p>Now we\u2019re onto the official Stage 1 of meiosis.&nbsp;<\/p>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>Meiosis Stage 1<\/strong><\/h4>\n\n\n\n<p>The replicated chromosomes line up in their pairs within the cell, and then they do this amazing thing where they swap bits of genetic information with each other. This is known as \u201ccrossing over\u201d, and the resulting chromosomes are called \u201crecombinant chromosomes\u201d. The existence of recombinant chromosomes is one of the reasons why there\u2019s such variety between offspring of the same parents.&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"241\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/crossing-over-e1587226898835.png\" alt=\"Meiosis Stage 1\" class=\"wp-image-1779\"\/><figcaption>With thanks to http:\/\/ibbiologyhelp.com\/Genetics\/crossover.png <\/figcaption><\/figure><\/div>\n\n\n\n<p>Super cute, huh?&nbsp;<\/p>\n\n\n\n<p>Next up, these pairs of replicated, crossed-over (recombinant) chromosomes are pulled in opposite directions from each other, with each of the pairs on opposite sides of the cell:&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"576\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/meiosis-1-1024x576.jpeg\" alt=\"Meiosis Stage 1\" class=\"wp-image-1786\"\/><figcaption>Imagine each of these chromosomes being pulled towards the outer ends of the cell. Note: there are just five replicated chromosomes in the image above. That\u2019s just so that it\u2019s easier to see what\u2019s going on.<br>With thanks to the Amoeba Sisters YouTube channel: https:\/\/www.youtube.com\/user\/AmoebaSisters<\/figcaption><\/figure><\/div>\n\n\n\n<p>The single cell reforms into two cells, each consisting of 23 replicated chromosomes. (23 chromosomes because \u2013 even though there are now 46 chromatids per cell rather than 92 \u2013 there are 23 centromeres. And remember: we count chromosomes according to how many centromeres there are.)<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"576\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/after-meiosis-1-1024x576.jpeg\" alt=\"Meiosis Stage 1 \u2013 part 2\" class=\"wp-image-1776\"\/><figcaption>Note: in the image above, there are just five replicated chromosomes in each cell. That\u2019s just so that it\u2019s easier to see what\u2019s going on.  With thanks to the Amoeba Sisters YouTube channel: https:\/\/www.youtube.com\/user\/AmoebaSisters<\/figcaption><\/figure>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>Meiosis Stage 2<\/strong><\/h4>\n\n\n\n<p>You have your two cells, each with 23 chromosomes. In each cell the chromosomes line up again, but this time they\u2019re in single file rather than pairs (their former \u201cpair\u201d, remember, is in the other cell).&nbsp;<\/p>\n\n\n\n<p>Each chromosome is separated, but this time it separates at the centre \u2013 at the centromere \u2013 essentially separating the original chromatid from the \u201creplicated\u201d chromatid (the one that was created during interphase).&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"576\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/meiosis-2-1024x576.jpeg\" alt=\"Meiosis Stage 2\" class=\"wp-image-1787\"\/><figcaption>With thanks to the Amoeba Sisters YouTube channel: https:\/\/www.youtube.com\/user\/AmoebaSisters<\/figcaption><\/figure><\/div>\n\n\n\n<p>The chromatids move to opposite ends of their respective cells, and the two cells reform into four cells.&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"576\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/after-meiosis-2-1024x576.jpeg\" alt=\"Meiosis Stage 2 \u2013 part 2\" class=\"wp-image-1777\"\/><figcaption>With thanks to the Amoeba Sisters YouTube channel: https:\/\/www.youtube.com\/user\/AmoebaSisters<\/figcaption><\/figure><\/div>\n\n\n\n<p>Each of the four cells is considered to have 23 chromosomes (rather than 11.5). Why? Because each chromosome retains a bit of centromere, and \u2013 as we now <em>definitely <\/em>know \u2013 chromosomes are counted according to how many centromeres there are.&nbsp;<\/p>\n\n\n\n<h4 class=\"wp-block-heading\"><strong>So at the end of each meiosis process, we end up with four sex cells<\/strong><\/h4>\n\n\n\n<p>Depending on whether they came from a male or a female, these sex cells are either four egg cells (i.e. four eggs) or four sperm cells (i.e. four sperm).&nbsp;<\/p>\n\n\n\n<p>Here\u2019s a lovely diagram to explain it all (showing just a few chromosomes to make it easier to understand):&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"269\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/meiosis-full-explanation-e1587304733871.jpg\" alt=\"Meiosis from start to finish\" class=\"wp-image-1788\"\/><figcaption>With thanks to https:\/\/www.philpoteducation.com\/mod\/book\/view.php?id=811#\/<\/figcaption><\/figure><\/div>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Tangent ends here<\/strong><\/h3>\n\n\n\n<hr class=\"wp-block-separator has-text-color has-background has-very-dark-gray-background-color has-very-dark-gray-color is-style-wide\"\/>\n\n\n\n<div style=\"height:50px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<p><strong>There\u2019s something I haven\u2019t told you \u2013 and it\u2019ll become super important later.<\/strong>..<\/p>\n\n\n\n<p>So you know meiosis Stage 1 and Stage 2? (If you didn\u2019t read the explanation above, just nod your head and pretend here.)&nbsp;<\/p>\n\n\n\n<p>In females, Stage 1 happens inside the eggs of a female foetus (!!!), which is impressively early and on-the-ball. (<a href=\"\/egg-quality-quantity-ovarian-reserve\/\">Women are born with all the eggs we\u2019ll ever have<\/a>: we don\u2019t produce eggs as we go along.) Stage 2 happens when a particular developing egg is about to ovulate. That means the two stages of meiosis within one egg can easily be decades apart.&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Back to our original question\u2026 Why are older women more likely to have egg cells with the incorrect number of chromosomes?&nbsp;<\/strong><\/h3>\n\n\n\n<p>We (by which I mean, scientists) are pretty certain that potential chromosome-numbering problems occur during meiosis. If you haven\u2019t read my massive tangent above, meiosis is the process by which egg cells (and sperm cells) are created.&nbsp;<\/p>\n\n\n\n<p>No one <em>really <\/em>knows what happens during meiosis to cause chromosome-numbering problems, but one of the main theories these days is to do with proteins. The theory goes like this:&nbsp;<\/p>\n\n\n\n<p>There are proteins present in chromosomes, which keep the chromatids of a chromosome nicely attached to each other. Lower levels of these proteins cause the chromatids to be more loosely connected.&nbsp;<\/p>\n\n\n\n<p>When the chromatids are more loosely connected, they\u2019re more likely to separate and \u201cstep out of line\u201d before it\u2019s required \u2013 i.e. before meiosis Stage 2 is fully underway. As a result, everything goes a bit wonky and the mature egg may end up with the wrong number of a particular chromosome. When the chromosomes don\u2019t separate as they\u2019re meant to, it\u2019s called \u201cnondisjunction\u201d. <\/p>\n\n\n\n<p>For example, the \u201cnondisjunctioned\u201d (not a word) egg might have two copies of Chromosome 21 as a result of not separating properly during meiosis. Once fused with a sperm cell, the resulting zygote may therefore end up with three copies of Chromosome 21, instead of the required two copies (one from the egg cell and one from the sperm cell).&nbsp;<\/p>\n\n\n\n<p><strong>What\u2019s all this got to do with older women? <\/strong><\/p>\n\n\n\n<p>Well, older women have lower amounts of protein in their eggs, suggesting that as the eggs age, the levels of these proteins fall. <\/p>\n\n\n\n<p>Caveat: this is all based on a study of <a href=\"https:\/\/www.nature.com\/articles\/ncomms15346\">mice<\/a>, but scientists looked at 16 human eggs donated from women who\u2019d done IVF to back up their research. They found that three eggs \u2013 all from women over 35 \u2013 were inclined to have the same \u201cearly separation\u201d problem too. Other studies have backed up these findings.&nbsp;<\/p>\n\n\n\n<p>It\u2019s probably safer to assume that the protein theory is just one potential reason why age affects chromosomes in eggs \u2013 although it is, admittedly, the most popular reason at present. As another study puts it, while we don\u2019t yet understand how age affects the likelihood of producing a normal egg, we can be fairly confident that there\u2019s more than one direct way in which maternal age affects egg quality. (That is, it\u2019s not <em>just<\/em> meiosis-related.) <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC2894811\/\">Here's more info on that study<\/a>.&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Can younger women have eggs with numerical abnormalities too?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Yes. All women have eggs with numerical abnormalities: it\u2019s just that older women tend to have <em>more <\/em>eggs with numerical abnormalities \u2013 which means it\u2019s more likely that a numerically abnormal egg will be fertilised.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>About structural abnormalities&nbsp;<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What makes a chromosome structurally abnormal?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Chromosomes contain strands of DNA in a highly organised way: there\u2019s nothing random when it comes to the way in which the DNA is arranged or ordered (or there shouldn\u2019t be, at least).&nbsp;<\/p>\n\n\n\n<p>When large sections of DNA are either missing from or added to a chromosome, it\u2019s referred to as a structural abnormality. There are a few different types, and they\u2019re all caused by what are known as \u201cmutations\u201d. Mutations are changes in our genetic material. They\u2019re often neutral and cause no bad effects, but some mutations are more harmful.&nbsp;<\/p>\n\n\n\n<p>As mentioned earlier, structural abnormalities are far less common than numerical ones.&nbsp;<\/p>\n\n\n\n<p>Here are the main types of mutation:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Deletion: when part of a chromosome is missing<\/strong>. Deletions are also known as partial monosomies, and they can occur in any part of any chromosome. A person with a deletion only has one copy of a particular chromosome segment instead of the usual two. (The usual two copies come from each of the two homologous chromosomes.)<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"572\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/deletion-e1587305328809.png\" alt=\"Chromosomal deletion\" class=\"wp-image-1780\"\/><figcaption>With thanks to https:\/\/www.yourgenome.org\/facts\/what-is-a-chromosome-disorder<\/figcaption><\/figure><\/div>\n\n\n\n<ul><li><strong>Duplication: when part of the chromosome is repeated, which results in extra genetic material<\/strong>. Duplications are also known as partial trisomies. <\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"770\" height=\"1024\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/duplication-770x1024.png\" alt=\"Chromosomal duplication\" class=\"wp-image-1782\"\/><figcaption>With thanks to https:\/\/www.yourgenome.org\/facts\/what-is-a-chromosome-disorder<\/figcaption><\/figure><\/div>\n\n\n\n<ul><li><strong>Translocation: when chromosomes are rearranged in an unusual way<\/strong>. This is the most common form of structural abnormality. There are two main types: <br><br>Reciprocal: when segments from two chromosomes are exchanged.&nbsp;<br><br>Robertsonian: when an entire chromosome from one pair attaches to a non-homologous chromosome (i.e. from another pair), resulting in a total of 45 chromosomes instead of 46.&nbsp;<br><br>Note: these are both different from \u201ccrossing over\u201d mentioned in meiosis earlier. Translocation occurs between non-homologous chromosomes (i.e. chromosomes from two different pairs), whereas crossing over occurs within one chromosomal pair. Crossing over is a normal process; translocation is not.&nbsp;<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"408\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/reciprocal-1024x408.png\" alt=\"Reciprocal translocation and Robertsonian translocation in chromosomes\" class=\"wp-image-1796\"\/><figcaption>With thanks to https:\/\/www.yourgenome.org\/facts\/what-is-a-chromosome-disorder<\/figcaption><\/figure><\/div>\n\n\n\n<ul><li><strong>Inversion: when a portion of a chromosome is inverted (in the opposite orientation)<\/strong>.&nbsp;<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"500\" height=\"572\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/inversion-e1587306923933.png\" alt=\"Chromosomal inversion\" class=\"wp-image-1785\"\/><figcaption>With thanks to https:\/\/www.yourgenome.org\/facts\/what-is-a-chromosome-disorder<\/figcaption><\/figure><\/div>\n\n\n\n<ul><li><strong>Ring: when a portion of chromosome has broken off and formed a ring\/circle shape<\/strong>.&nbsp;<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"910\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/ring-1024x910.png\" alt=\"Ring chromosome\" class=\"wp-image-1800\"\/><figcaption>With thanks to https:\/\/www.yourgenome.org\/facts\/what-is-a-chromosome-disorder<\/figcaption><\/figure><\/div>\n\n\n\n<p>All these structural abnormalities can be divided into two categories:&nbsp;<\/p>\n\n\n\n<ul><li><strong>Balanced: the complete chromosomal set is still present, even though it\u2019s rearranged<\/strong>. Includes inverted and reciprocal translocated chromosomes, as well as some Robertsonian chromosomes. (Robertsonian chromosomes can be balanced if all the genetic information is still available \u2013 even if you have the wrong number of chromosomes).&nbsp;<\/li><li><strong>Unbalanced: there\u2019s extra or missing information<\/strong>. Includes deletions, duplications, insertions, some Robertsonian translocations, and ring chromosomes.&nbsp;<\/li><\/ul>\n\n\n\n<p>Unbalanced abnormalities tend to be more serious, and balanced ones can actually go undetected sometimes. (The full set of DNA material is still present, so there may not be any symptoms.)&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What kinds of problems do these structural abnormalities cause?&nbsp;<\/strong><\/h3>\n\n\n\n<p>As with numerical abnormalities, it depends on the chromosome affected. Unlike numerical abnormalities, there are many structural abnormalities that <em>don\u2019t <\/em>lead to miscarriage or infant death (although symptoms can still be severe and difficult to manage).&nbsp;<\/p>\n\n\n\n<p>Below are some of the main types of structural abnormalities. I haven\u2019t gone into detail on the symptoms, but I\u2019ve provided links to more information on each one. Symptoms often involve distinctive facial\/bodily characteristics and learning disabilities, among others.&nbsp;<\/p>\n\n\n\n<ul><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/7896\/wolf-hirschhorn-syndrome\"><strong>Wolf-Hirschhorn syndrome<\/strong><\/a> is caused by a deletion on the short arm of chromosome 4.&nbsp;<\/li><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/307\/jacobsen-syndrome\"><strong>Jacobsen syndrome<\/strong><\/a> is caused by a deletion on the long arm of chromosome 11.&nbsp;<\/li><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/6213\/cri-du-chat-syndrome\"><strong>Cri-du-chat syndrome<\/strong><\/a> is caused by a deletion on the small arm of chromosome 5.&nbsp;<\/li><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/10299\/22q112-deletion-syndrome\"><strong>22q11.2 deletion syndrome<\/strong><\/a> is caused by a deletion of a small part of chromosome 22 near the middle of the chromosome at a location known as q11.2.<\/li><li>In less than 5% of cases, <a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/10247\/down-syndrome\"><strong>Down syndrome<\/strong><\/a> is caused by a Robertsonian translocation of the long arm of chromosome 21 onto the long arm of chromosome 14. (Basically, the Robertsonian translocation causes a \u201ctrisomy 21\u201d \u2013 which is a numerical abnormality.)<\/li><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/399\/xx-male-syndrome\"><strong>XX male syndrome<\/strong><\/a> is caused by a translocation of a particular gene from the Y to the X chromosome.&nbsp;<\/li><li><a href=\"https:\/\/rarediseases.info.nih.gov\/diseases\/1328\/ring-chromosome-15\"><strong>Ring chromosome 15<\/strong><\/a> occurs when chromosome 15 breaks and the broken ends fuse together.&nbsp;<\/li><\/ul>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter size-large is-resized\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x.png\" alt=\"\" class=\"wp-image-492\" width=\"229\" height=\"105\" srcset=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x.png 443w, https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1285@2x-300x137.png 300w\" sizes=\"(max-width: 229px) 100vw, 229px\" \/><\/figure><\/div>\n\n\n\n<p><\/p>\n\n\n<style>.elementor-1039 .elementor-element.elementor-element-6e3b2cc{margin-top:2%;margin-bottom:12%;padding:1% 4% 1% 4%;z-index:2;}.elementor-1039 .elementor-element.elementor-element-cab4a86 > .elementor-element-populated{margin:0% 5% 0% 5%;--e-column-margin-right:5%;--e-column-margin-left:5%;}.elementor-1039 .elementor-element.elementor-element-4a9f61d:not(.elementor-motion-effects-element-type-background), .elementor-1039 .elementor-element.elementor-element-4a9f61d > .elementor-motion-effects-container > .elementor-motion-effects-layer{background-color:#FCFAEE;}.elementor-1039 .elementor-element.elementor-element-4a9f61d{transition:background 0.3s, border 0.3s, border-radius 0.3s, box-shadow 0.3s;}.elementor-1039 .elementor-element.elementor-element-4a9f61d > .elementor-background-overlay{transition:background 0.3s, border-radius 0.3s, opacity 0.3s;}.elementor-1039 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*\/<\/style>\t\t<div data-elementor-type=\"section\" data-elementor-id=\"1039\" class=\"elementor elementor-1039\" data-elementor-post-type=\"elementor_library\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-top-section elementor-element elementor-element-6e3b2cc elementor-section-stretched elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"6e3b2cc\" data-element_type=\"section\" data-settings=\"{&quot;stretch_section&quot;:&quot;section-stretched&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-cab4a86\" data-id=\"cab4a86\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-inner-section elementor-element elementor-element-4a9f61d elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"4a9f61d\" data-element_type=\"section\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-inner-column elementor-element elementor-element-f69664e\" data-id=\"f69664e\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-53798fd elementor-align-center elementor-widget elementor-widget-button\" data-id=\"53798fd\" data-element_type=\"widget\" data-widget_type=\"button.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<div class=\"elementor-button-wrapper\">\n\t\t\t<a class=\"elementor-button elementor-size-sm\" role=\"button\" id=\"fdg\">\n\t\t\t\t\t\t<span class=\"elementor-button-content-wrapper\">\n\t\t\t\t\t\t<span class=\"elementor-button-text\">FREE EMAIL<\/span>\n\t\t<\/span>\n\t\t\t\t\t<\/a>\n\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-6567c58 elementor-widget elementor-widget-heading\" data-id=\"6567c58\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<p class=\"elementor-heading-title elementor-size-default\">One embryo or two?<\/p>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b952531 elementor-widget elementor-widget-text-editor\" data-id=\"b952531\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p>What are the risks and benefits of transferring more than one embryo? When clinics and hospitals recommend a single embryo transfer, do they have an underlying incentive?<\/p><p>What are the real facts?<\/p><p>I dug deep into all the research so you don't have to! Enter your email below for an easy-to-read, easy-to-understand guide to &#8220;one embryo vs two&#8221;.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-df03e60 elementor-mobile-button-align-center elementor-tablet-button-align-stretch elementor-button-align-stretch elementor-widget elementor-widget-form\" data-id=\"df03e60\" data-element_type=\"widget\" data-settings=\"{&quot;button_width&quot;:&quot;20&quot;,&quot;button_width_mobile&quot;:&quot;100&quot;,&quot;button_width_tablet&quot;:&quot;25&quot;,&quot;step_next_label&quot;:&quot;Next&quot;,&quot;step_previous_label&quot;:&quot;Previous&quot;,&quot;step_type&quot;:&quot;number_text&quot;,&quot;step_icon_shape&quot;:&quot;circle&quot;}\" data-widget_type=\"form.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<form class=\"elementor-form\" method=\"post\" name=\"New Form\">\n\t\t\t<input type=\"hidden\" name=\"post_id\" value=\"1039\"\/>\n\t\t\t<input type=\"hidden\" name=\"form_id\" value=\"df03e60\"\/>\n\t\t\t<input type=\"hidden\" name=\"referer_title\" value=\"The Duff\" \/>\n\n\t\t\t\n\t\t\t<div class=\"elementor-form-fields-wrapper elementor-labels-\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-field-type-email elementor-field-group elementor-column elementor-field-group-email elementor-col-80 elementor-md-75 elementor-sm-100 elementor-field-required\">\n\t\t\t\t\t\t\t\t\t\t\t\t<label for=\"form-field-email\" class=\"elementor-field-label elementor-screen-only\">\n\t\t\t\t\t\t\t\tEmail\t\t\t\t\t\t\t<\/label>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t<input size=\"1\" type=\"email\" name=\"form_fields[email]\" id=\"form-field-email\" class=\"elementor-field elementor-size-lg  elementor-field-textual\" placeholder=\"Your email address\" required=\"required\" aria-required=\"true\">\n\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t\t\t<div class=\"elementor-field-group elementor-column elementor-field-type-submit elementor-col-20 e-form__buttons elementor-md-25 elementor-sm-100\">\n\t\t\t\t\t<button type=\"submit\" class=\"elementor-button elementor-size-lg\">\n\t\t\t\t\t\t<span >\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<span class=\" elementor-button-icon\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-button-text\">Get the facts<\/span>\n\t\t\t\t\t\t\t\t\t\t\t\t\t<\/span>\n\t\t\t\t\t<\/button>\n\t\t\t\t<\/div>\n\t\t\t<\/div>\n\t\t<\/form>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-861ee7c elementor-widget elementor-widget-text-editor\" data-id=\"861ee7c\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><strong>I won't send you spam. Unsubscribe at any time.<\/strong><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t<\/div>\n\t\t\n\n\n<h3 class=\"wp-block-heading\"><strong>What causes these structural abnormalities to occur?&nbsp;<\/strong><\/h3>\n\n\n\n<p>As with numerical abnormalities, many structural abnormalities are a result of mistakes during the meiosis or mitosis stages. And (just as with numerical abnormalities) meiosis-related mistakes are more common \u2013 and \u201cadvanced maternal age\u201d makes it more likely that these mistakes will occur.&nbsp;<\/p>\n\n\n\n<p>But <em>unlike<\/em> numerical abnormalities, many structural abnormalities are also inherited (and not associated with maternal age).&nbsp;<\/p>\n\n\n\n<p>To give one example of how an abnormality might be inherited&#8230; the chromosomes in your own body\u2019s cells might be balanced reciprocal translocated chromosomes \u2013 that is, the segments from two chromosomes have broken off and swapped places with each other. You still have the same number of chromosomes as before and you still have all the genetic information you need \u2013 it\u2019s just in a slightly different place compared to other people. You show no symptoms and live a normal, healthy life.<\/p>\n\n\n\n<p>BUT because your chromosomes aren\u2019t exactly where they\u2019re meant to be, you\u2019re considered a \u201ccarrier\u201d. When you conceive, your embryo will only inherit half your chromosomes (the other half come from the father). If the father has \u201cnormal\u201d chromosomes, there are four possible outcomes for any potential embryo:&nbsp;<\/p>\n\n\n\n<div class=\"wp-block-image Mish\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"723\" height=\"1024\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/04\/Balanced-translocations-FTNP-dragged-723x1024.png\" alt=\"Potential outcomes if you have balanced translocations\" class=\"wp-image-1778\"\/><figcaption>With thanks to www.rarechromo.org<\/figcaption><\/figure><\/div>\n\n\n\n<p>As described in the image above, a baby with unbalanced chromosomes has a high likelihood of being miscarried. If it survives pregnancy, it will have special needs and health problems.&nbsp;<\/p>\n\n\n\n<p>It\u2019s important to be aware that if you\u2019re a balanced translocation carrier, it doesn\u2019t mean there\u2019s a one in four chance that embryos created by you and your partner will have unbalanced chromosomes. As <a href=\"https:\/\/academic.oup.com\/humrep\/article\/17\/12\/3201\/569611\">one study<\/a> found, \u201cThe incidence of abnormal pregnancies in carriers of balanced translocations depends strictly on the chromosomes involved in the translocations.\u201d&nbsp;<\/p>\n\n\n\n<p>Translocations are just one example of structural abnormalities that can <strong>either<\/strong> be inherited <strong>or<\/strong> a result of mitosis\/meiosis.&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>How do I find out if I\u2019m a carrier of inherited chromosomal abnormalities?&nbsp;<\/strong><\/h3>\n\n\n\n<p>If you have problems getting pregnant or staying pregnant, it might<em> <\/em>indicate you\u2019re a carrier \u2013 because the fertilised eggs aren\u2019t surviving long. There are, of course, many other reasons why people struggle to get pregnant, so you\u2019ll need to speak to your doctor about any tests and investigations you could have.&nbsp;<\/p>\n\n\n\n<p>If you think you or your partner might be a carrier, you can get tested before trying for a baby: it\u2019s just a simple blood test that involves examining some of your blood cells.&nbsp;<\/p>\n\n\n\n<p>If you become pregnant with a baby and prenatal screening tests show that the baby has a chromosomal condition (like Down syndrome), it <em>might <\/em>indicate you\u2019re a carrier \u2013 although there\u2019s also a chance the condition is due to errors in meiosis or mitosis.&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>If I find out I\u2019m a carrier, what are my options if I want to have a baby?&nbsp;<\/strong><\/h3>\n\n\n\n<p>You could try to get pregnant the \u201cregular\u201d way, without any medical help. (Or if you need fertility treatment, you could try to conceive through IVF, ICSI, etc.) If\/when you get pregnant, you could then decide whether or not to have a prenatal test to determine if your baby is likely to develop the chromosomal abnormality in question.&nbsp;<\/p>\n\n\n\n<p>Note: this prenatal test will be different from the regular prenatal screening test that everyone is offered on the NHS.<\/p>\n\n\n\n<p>Or you could have something called \u201cpreimplantation genetic diagnosis\u201d (PGD) on your embryos. You\u2019d need to do IVF, though: you couldn\u2019t conceive naturally. More on PGD in a sec.&nbsp;<\/p>\n\n\n\n<p>Or you might prefer to use healthy donor eggs or sperm instead. Adoption, of course, is always an option too.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>About chromosomal abnormalities in sperm<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Can sperm cells have numerical or structural abnormalities too?&nbsp;<\/strong><\/h3>\n\n\n\n<p>Thank you for the reminder! Yes: a proportion of every man\u2019s sperm will be aneuploid. But whereas women are born with all the eggs they\u2019ll ever have (see my tangent on meiosis above for why this contributes to abnormalities), men produce new sperm continually. A 40-year-old man doesn\u2019t have 40-year-old sperm. Instead he has \u2013 at most \u2013 <a href=\"\/sperm-collection-ivf\/\">three-month-old sperm<\/a>.&nbsp;<\/p>\n\n\n\n<p>But that doesn\u2019t mean all men will have a similar proportion of euploid and aneuploid sperm. Some men will have a higher proportion of aneuploid sperm than others, and researchers have found it <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3735152\/\">difficult<\/a> to provide an \u201caverage\u201d percentage figure.&nbsp;<\/p>\n\n\n\n<p>As for what causes sperm aneuploidy\u2026 that\u2019s a question for another article when I\u2019ve done more research. One thing I discovered on my journeys across the internet, though (and which I may as well mention now), is that there <em>might <\/em>be a link between sperm aneuploidy and \u201cteratozoospermia\u201d.&nbsp;<\/p>\n\n\n\n<p>Yup, that\u2019s a real word \u2013 and you may be familiar with it if your partner has already been tested. Teratozoospermia is an increase in the percentage of abnormally shaped sperm in a sperm sample (anything above 4% is generally classified as teratozoospermia), and it\u2019s one of the most common causes of male subfertility. Studies (like <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pubmed\/12524092\">this one<\/a>) suggest that men with teratozoospermia are more likely to have aneuploid sperm.&nbsp;<\/p>\n\n\n\n<p>Other studies (like <a href=\"https:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3735152\/\">this one<\/a>) link sperm aneuploidy with all types of male subfertility: oligozoospermia (low concentration of sperm in semen), asthenozoospermia (poor motility\/ability to move around) and teratozoospermia (poor morphology).&nbsp;<\/p>\n\n\n\n<p>When you read my article about <a href=\"\/pgs-testing-ivf\/\">preimplantation genetic screening (PGS)<\/a>, I discuss it mainly in the context of women who are older or have a history of miscarriage. But it might also be worth considering if your male partner is known to have subfertility issues.&nbsp;<\/p>\n\n\n\n<p>The overwhelming number of embryo abnormalities, though, are due to errors from the female parent.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Can I find out if my embryos have any chromosomal abnormalities (numerical or structural)?&nbsp;<\/strong><\/h2>\n\n\n\n<p>Yes: embryos can be tested before they\u2019re selected for transfer into your uterus. (This is a big benefit of IVF over natural conception if you\u2019re older or a carrier of a chromosomal abnormality.) There are two types of test:&nbsp;<\/p>\n\n\n\n<ul><li><strong><a href=\"\/pgs-testing-ivf\/\">PGS (preimplantation genetic screening)<\/a> tests for numerical abnormalities <\/strong>\u2013 which are the most common type of abnormality, and which disproportionately affect older women. <br><br>Numerical abnormalities generally aren\u2019t inherited. <br><br>As mentioned earlier, it\u2019s possible that an embryo could have a <em>structural<\/em> abnormality that isn\u2019t<em> <\/em>inherited. PGS can pick up on some of these abnormalities too. <br><br><a href=\"\/pgs-testing-ivf\/\">Read much more about PGS here<\/a>.&nbsp;<\/li><\/ul>\n\n\n\n<ul><li><strong>PGD (preimplantation genetic diagnosis) tests for structural abnormalities <\/strong>\u2013 which are less common, and which are often inherited. PGD is usually only offered to people who are known carriers of structural abnormalities.<br><br>I\u2019ll have a new article about PGD available soon.&nbsp;<\/li><\/ul>\n\n\n\n<p>FYI PGS and PGD have recently been re-branded! <\/p>\n\n\n\n<ul><li>PGS is now PGT-A, where \u201cPGT\u201d stands for \u201cpreimplantation genetic testing\u201d and \u201cA\u201d stands for \u201caneuploidy\u201d. <br><\/li><li>PGD, meanwhile, has been divided into two new terms:&nbsp;<br><br>PGT-M (\u201cmonogenic\u201d\/single cell) looks for specific inherited conditions controlled by a single gene (like Huntington\u2019s disease, sickle-cell disease and Fragile X syndrome) \u2013 a topic not covered by this article.&nbsp;<br><br>PGT-SR (\u201cstructural rearrangements\u201d) looks for specific inherited chromosomal abnormalities (like translocations and inversions).&nbsp;<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Next up: read all about PGS<\/strong><\/h2>\n\n\n\n<p>Congratulations: I'm pretty certain you're in the top 1% of the population when it comes to understanding chromosomes! <\/p>\n\n\n\n<p>Now go put that knowledge to good use: read up on how preimplantation genetic screening (PGS) works, find out the pros and cons, and reach a decision about whether you should shell out for it. <\/p>\n\n\n\n<p><a href=\"\/pgs-testing-ivf\/\">Read the article about PGS testing here<\/a>. <\/p>\n\n\n\n<p>(And I promise I'll write about PGD soon!)<\/p>\n\n\n<style>.elementor-536 .elementor-element.elementor-element-3a6436ac{padding:4% 22% 8% 22%;z-index:2;}.elementor-536 .elementor-element.elementor-element-17d4eb46:not(.elementor-motion-effects-element-type-background) > .elementor-widget-wrap, .elementor-536 .elementor-element.elementor-element-17d4eb46 > .elementor-widget-wrap > .elementor-motion-effects-container > .elementor-motion-effects-layer{background-color:#C0E4E5;}.elementor-536 .elementor-element.elementor-element-17d4eb46 > 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.elementor-element.elementor-element-3d61ba35 .elementor-post__title a{font-size:1em;}}@media(min-width:768px){.elementor-536 .elementor-element.elementor-element-ef00547{width:13%;}.elementor-536 .elementor-element.elementor-element-19a555d6{width:87.031%;}}@media(max-width:1024px) and (min-width:768px){.elementor-536 .elementor-element.elementor-element-ef00547{width:100%;}.elementor-536 .elementor-element.elementor-element-19a555d6{width:100%;}}<\/style>\t\t<div data-elementor-type=\"section\" data-elementor-id=\"536\" class=\"elementor elementor-536\" data-elementor-post-type=\"elementor_library\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-top-section elementor-element elementor-element-3a6436ac elementor-section-stretched elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"3a6436ac\" data-element_type=\"section\" data-settings=\"{&quot;stretch_section&quot;:&quot;section-stretched&quot;}\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-17d4eb46\" data-id=\"17d4eb46\" data-element_type=\"column\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-inner-section elementor-element elementor-element-40f8e482 elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"40f8e482\" data-element_type=\"section\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-50 elementor-inner-column elementor-element elementor-element-ef00547\" data-id=\"ef00547\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-576153c1 elementor-widget elementor-widget-image\" data-id=\"576153c1\" data-element_type=\"widget\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<img loading=\"lazy\" decoding=\"async\" width=\"94\" height=\"142\" src=\"https:\/\/theduff.co.uk\/wp-content\/uploads\/2020\/02\/Group-1294@2x.png\" class=\"attachment-large size-large wp-image-493\" alt=\"Lighbulb illustration\" \/>\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t<div class=\"elementor-column elementor-col-50 elementor-inner-column elementor-element elementor-element-19a555d6\" data-id=\"19a555d6\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-574ef11f elementor-widget elementor-widget-heading\" data-id=\"574ef11f\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<p class=\"elementor-heading-title elementor-size-default\">You might also like to read&#8230;<\/p>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-3d61ba35 elementor-grid-1 elementor-posts--thumbnail-none elementor-grid-tablet-1 elementor-grid-mobile-1 elementor-widget elementor-widget-posts\" data-id=\"3d61ba35\" data-element_type=\"widget\" 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exams but didn\u2019t understand your teacher\u2019s explanations\u2026 this page isn\u2019t quite intended for you, but<\/p>\n","protected":false},"author":2,"featured_media":2307,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,9],"tags":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v21.6 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Chromosomal abnormalities and IVF: what you need to know<\/title>\n<meta name=\"description\" content=\"This article will give you a thorough understanding of your embryos, how chromosomal defects occur, who\u2019s at risk, and lots more.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/theduff.co.uk\/chromosomal-abnormalities-aneuploidy-ivf\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta 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