Newborn, baby, toddler, child, tweenager, teenager, young adult, adult, middle-aged, no spring chicken, elderly… we love to give different names to humans as they get older and develop in some way. And it’s no different for pre-newborn stages, either.

Here’s what you need to know:

Fertilisation

Fertilised egg in fallopian tube

When an egg is fertilised by sperm in the fallopian tube (or petri dish), it becomes a zygote. The zygote is a single cell that contains all 46 of the chromosomes needed to become a fully fledged human. A zygote is pretty much the first stage of human life.

Zygote(If you want to learn about pre-zygote terminology – like egg, oocyte, etc. – read this.)

24 hours after fertilisation

The new cell divides in two for the first time and it’s no longer called a zygote. Now it’s an embryo (and you thought your twenties went by too quickly…). It continues its journey through the fallopian tube towards the uterus, and divides in two yet again.

 

Two-cell embryo and four-cell embryo

 

Days 3–4 after fertilisation

Providing you have a high-quality embryo, the cells of the embryo keep dividing in two every day as it travels through the fallopian tube – and for the first three or four of those days it’s called a morula. So it’s both an embryo and a morula – in the same way that a teenager is both a teenager and a pain in the butt.

Morula

Keep reading… there's more below.

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Day 5 after fertilisation

The embryo is no longer referred to as a morula: it’s a blastocyst instead. At this point, a cavity forms inside the embryo – and it’s inside this cavity that a baby’s body will eventually form. The embryo will also break down its protective outer membrane, enabling it to enter the uterus from the fallopian tube – where it will hopefully “implant” and continue to grow and develop. (This is known as “hatching”.)

BlastocystIf you’re undergoing IVF, you’ll most likely be doing a “five-day embryo transfer”. This means that all the stuff mentioned above will have happened in the lab rather than in your fallopian tube, and then the blastocyst will be transferred directly into your uterus on day 5. (If you’re freezing your embryos for later use, the embryos will be frozen on day 5.)

Day 7–8 after fertilisation

“Embryo implantation” happens. This is when the embryo – at blastocyst stage – fixes itself to an inner layer of the uterus called the endometrium. Not all embryos will be able to implant – due to both medical factors and pure luck – but those that do will take a further seven days or so to complete the implantation process.

Embryo implantation

Day 14 after fertilisation

The embryo has finally finished implanting itself properly, and the word “pregnant” can (also finally) be used. From now on, the embryo grows rapidly. Over time, it starts to look less like a ball of cells and more like a (rather odd and strangely proportioned) baby.

Week 8 after fertilisation

It’s time to start calling it a foetus! Why now? Why eight weeks? Because the embryo has developed to such an extent that the body’s major organs (like the brain, liver and kidneys) are able to form. It’s an occasion that clearly warrants a name change – if not a chocolate, hummus & smoked salmon-themed celebratory party. The foetus will continue to be called a foetus until it’s born. And then it’ll become a “newborn” or “that cute thing that cries, poops and pukes all the time”.

Foetus

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