EMBRYO, EMBY, EMBABY, JOHN… whatever you choose to call those little multicellular diploid eukaryotic organisms, you can be certain they’re going to take up a LOT of your headspace as you go through IVF.
You’ll have questions like…
- Why do some clinics transfer or freeze Day 3 embryos, while others transfer or freeze Day 5 embryos?
- Which is better?
- Why do some people talk about Day 6 transfers?
- Why does no one ever want to transfer (or freeze) a Day 4 embryo?
This article attempts to answer them all!
A quick note about embryo grading…
You'll probably also want to know about embryo grading for Day 3 and Day 5 embryos. How is embryo grading performed? What does it tell us? Can it be trusted? What's a “good” embryo grade? What's a “bad” embryo grade? And so on. There's a separate article on all that called “Embryo grading for Day 3 and Day 5 embryos“.
Read this first!
The stages of embryo development
Everything in this article needs to be understood in the context of the development of the embryo – otherwise it just won’t make sense. So allow me to take you on a fascinating tour of the contents of your doctor’s petri dish…
When an egg is fertilised by sperm (whether in the fallopian tube or the 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.
24 hours after fertilisation
The new cell divides (“cleaves”) in two for the first time and it’s no longer called a zygote: it’s an “embryo” now. If you ever read about “cleavage stage” embryos, it’s this stage – and the stage ends when the embryo becomes a “blastocyst” later on.
In a “natural” pregnancy, this embryo continues its journey through the fallopian tube towards the uterus, and divides in two yet again. In IVF cases, this all happens in the petri dish.
Day 2 after fertilisation
The cells of the embryo continue to divide in two. (In normal-developing embryos, cells divide every 18 to 20 hours.)
Each of the cells is called a “blastomere” – and although they’re dividing and therefore increasing in number, the embryo itself remains about the same size throughout the cleaving process. (Think of it like dividing up a pizza into two slices, then four slices, then eight slices.)
Small portions of the cells may break off as they’re dividing; this is called “fragmentation”, and it can negatively affect the quality of the embryo. More on this later.
In a “natural” pregnancy, all this happens while the embryo travels through the fallopian tube towards its final resting place in the uterus. With IVF, it happens in the petri dish.
Day 3 after fertilisation
The embryo is still busy cleaving at this stage.
Depending on the clinic and the patient, some embryos are frozen or transferred on Day 3 – at this “cleavage stage” of development. If not, the embryo continues to develop in the petri dish…
Day 4 after fertilisation
The embryo goes through something called “compaction”, which is when the cells bind together tightly into a sphere. It’s difficult to make head or tail of anything when looking at an embryo at this point, but that’s fine and expected. It’s now called a “morula”, which is Latin for “mulberry” (because that’s kind of what it looks like).
If compaction doesn’t start on Day 4, it’s less likely that the embryo will develop into a blastocyst (see next step).
Day 5 / Day 6 after fertilisation
The embryo is now referred to as a “blastocyst”, and it’s no longer dividing (“cleaving”). It consists of anywhere between 80 and 200+ cells, but no one’s counting anymore because there are more interesting things to report. (And also because, I imagine, no one wants to sit there counting to 200 a few dozen times a day.)
Fluid accumulates inside the blastocyst, causing it to expand in size, and the cells start to organise themselves into two groups: the “inner cell mass” and the “trophectoderm”. There are therefore three distinct parts to the blastocyst:
- The inner cell mass (ICM) – which is made up of cells that will go on to form the foetus
- The trophectoderm (TE) – which is a layer of cells that will go on to become the placenta
- The blastocoel, which is the fluid-filled cavity of the blastocyst
Here’s what it looks like:
Here’s what it looks like in real life (“C” is the fluid-filled cavity, “T” is the trophectoderm, and “ICM” is the inner cell mass):
Note: some embryos are super keen and will reach blastocyst stage at Day 4. Embryologists reckon that blastocysts that form earlier (Day 4 or Day 5) have a higher likelihood of implanting than those that form later (Day 6 onwards) because they seem most “eager” to implant. That doesn’t mean a Day 6 blastocyst won’t result in a healthy pregnancy: not all embryos develop at the exact same time as each other, and many of them will still implant.
Anyway! Back to our blastocyst. As it expands in size, the blastocyst also breaks down its protective outer membrane (called the “zona pellucida”), to enable it to implant properly once it’s in the uterus. This process is known as “hatching”.
Many embryos these days are frozen or transferred at this “blastocyst stage”. Some embryos will have already begun to hatch, while others will be on the verge of hatching.
These images show the hatching process in action:
Day 5 blastocyst (pre-hatching)
HATCHED and ready to implant! (No more zona pellucida)
Day 7 / Day 8 after fertilisation
“Embryo implantation” happens. This is when the blastocyst 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.
Transferring Day 3 vs Day 5 embryos
In the past, almost all embryos would be transferred (or frozen) three days after successful fertilisation. Today, many clinics prefer to transfer or freeze the embryos five or six days after fertilisation instead – at blastocyst stage.
There are many reasons why clinics prefer to transfer/freeze embryos at blastocyst stage:
- By the time the embryo has become a blastocyst, it’s already survived a ton of developmental obstacles and is therefore more likely to survive the perilous journey from the doctor’s petri dish to your uterus. (It’s also more likely to survive the freeze-thaw process if it’s being frozen.)
- The embryo should have a stronger chance of implanting because it’s a more natural time for the embryo to be in the uterus. (In a “natural” pregnancy, the embryo would still be travelling down the fallopian tube at Day 3 – and would reach the uterus around Day 5.)
- As mentioned above, embryos that become blastocysts by Day 5 are more likely to result in healthy pregnancies than embryos that develop later. An embryologist can only know which are the “speedy” blastocysts by monitoring them in the petri dish past Day 3.
- “Embryo grading” (a visual assessment of the embryo’s “morphology”, or appearance) is considered more accurate at Day 5 – when there’s more to see and assess. The thinking goes that a Day 3 embryo could look fabulous (or atrocious), but there isn’t enough information to go on – so Day 3 embryo grading might result in a sub-optimal embryo being transferred.
If you're interested, I have an entire article about embryo grading.
- Another reason for favouring Day 5 embryos is more tangentially related to success rates: more and more clinics are experimenting with “pre-implantation genetic screening” (PGS), which in theory allows them to identify “competent” embryos that contain all 46 chromosomes. Clinics strongly prefer doing PGS on Day 5 embryos rather than Day 3 embryos. (You can read more about why that’s the case here.)
There’s quite a lot of controversy around whether PGS actually helps to improve pregnancy/birth rates. You can read all about it here.
So… clinics’ reasons for preferring Day 5 embryos sounds perfectly plausible in theory – to me, at least – but is there any evidence that transferring Day 5 embryos is more successful?
Not all clinics agree with performing Day 5 transfers, however – and many continue to transfer (or freeze) Day 3 embryos for some or all patients. Their reasons are as follows:
- Some clinics believe that the uterus is a better and/or more natural environment for the embryo to grow and develop – so it’s better for it to be moved there as soon as possible.
- Linked to the above point… it’s perfectly possible that an embryo wouldn’t survive to Day 5 in the petri dish but could survive in the uterus if transferred at Day 3.
- And linked to point (2)… If a couple ends up with just one or two embryos after IVF treatment, the clinic may decide it’s better to transfer the embryo ASAP rather than hope and pray that it’ll survive a couple more days in the petri dish. (I.e. it’s better to put in a younger embryo than none at all.)
- Some clinics might feel they don’t have the most up-to-date technology to provide the best environment for growing (“culturing”) an embryo to Day 5.
And just as you’d expect, some studies (like this one) show that transferring Day 5 embryos isn’t better than transferring Day 3 embryos.
Questions to ask your doctor at every stage of IVF treatment: free downloadable guide
IVF isn’t just overwhelming; it can also be a mind-boggling and sometimes terrifying experience because there’s so much to learn and so many rules to follow.
Problem is, doctors are busy – and they often don’t have the time to anticipate your concerns and provide all the information you might need.
This downloadable guide contains questions that will help you understand the process better, get the answers you deserve, and feel more in control of the situation (and your rights as a patient).
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Why aren’t Day 4 embryos transferred?
Here’s what a Day 4 embryo looks like compared to all the other days:
Yes: it’s a bit of a shitshow. It’s just a mass of cells with no distinct features.
This is the “compaction” stage I referred to earlier – and it’s when the cells bind together tightly into a sphere. It’s hard to figure out what the heck is going on with Day 4 embryos: they can’t be “graded” using the methods used to grade Day 3 and Day 5 (or Day 6) embryos, so it’s therefore difficult to decide whether they stand a chance of successful implantation. As a result, embryologists tend to leave them well alone.
BUT some clinics (like this one in Australia) have reported success with creating their own “grading” system for Day 4 embryos (as well as success in transferring them). The main benefits of being able to transfer Day 4 embryos are that it gives doctors and patients more flexibility and opportunities to transfer an embryo. Clinics in the UK might follow suit one day; we’ll have to wait and see.
Now learn about embryo grading
Embryo grading is a largely subjective assessment of the “morphology” (the visual appearance) of embryos under a microscope. Read my article about embryo grading if you want to learn the following:
- What's the purpose of embryo grading?
- What does it mean when an embryo has a good grade?
- Does embryo grading discover chromosomal abnormalities too?
- Day 3 embryo grading explained
- Deciphering and understanding Day 3 embryo grades
- Day 5 embryo grading explained
- Deciphering and understanding Day 5 embryo grades
- What's the “perfect” grade?
- What’s the lowest grade an embryologist would consider transferring?