Ask an Embryologist

Embryology is a field that most people haven’t heard of…unless they’ve gone through IVF. And for those of us who have been through IVF, odds are we’ve had a LOT of questions about embryos. We asked our Instagram followers to send us their big questions about embryos and reached out to Kristen Jones, an embryologist in Australia to help us answer them!

How did you first hear about embryology as a profession? What drew you to the field?

I was intrigued by IVF in my teenage years as I had family members go through it but I assumed it was for doctors. During my science degree we had to do an internship and I found out that an IVF lab was an option. So I jumped at that and loved it.


What does an embryologist actually do? What does a typical work day look like?

An embryologist is a scientist that deals with the eggs, sperm and embryos. We process the semen samples whether that be for a semen analysis, IUI, IVF or ICSI. In the embryology lab we do the egg collections (find the eggs in the fluid), inseminate the eggs, check the embryos, freeze/thaw/prepare media/dishes and make sure the lab is running effectively.


What do the different embryo grades mean?

Embryo grading is referring to how we assess the morphology or appearance of an embryo. Different labs have different grading systems, but essentially we all look for the same things but just have different codes to write them down. 

For a day 3 embryos we assess how many cells, how even the cells are/how nice they look and also how much fragmentation is present.

For a blastocyst (day 5/6/7) we assess the size or expansion of the cavity, and we grade the inner cell mass (ball of cells inside the cavity) and trophectoderm which are all of the surrounding cells.

We do our best to pick the embryo we feel has the highest implantation potential but it’s important to note that the most amazing looking embryo can fail and a poor one can work.


How do embryologists pick the best embryo to transfer?

Using the grading system as described above we choose the highest graded embryo. Sometimes we have several to choose from that are all the same quality. In these situations we will look back on previous development to see if one looked nicer at a previous check. Some labs use timelapse monitoring and software which analyses the footage and gives an estimate of the embryos with the highest implantation potential.


Can a batch of embryos look good (euploid, good morphology) but still be non-viable?

Yes, unfortunately good quality euploid embryos are not guaranteed to work. There is nothing in IVF with a success rate of 100% unfortunately.


What types of genetic testing exist? What’s the difference between PGD and PGS?

PGT-A is preimplantation genetic testing for aneuploidy and was formally known as PGS (preimplantation genetic screening). This is the screening of an embryo’s chromosomes. This may be suggested by your doctor in cases of recurrent pregnancy loss, advanced maternal age, repeated implantation failure.

PGD is preimplantation genetic diagnosis and is where we want to test for a specific disease such as cystic fibrosis. This is now known as PGT-M preimplantation genetic testing for monogenic/single gene disorders. This would be used in cases where there is a known condition. 

There is also PGT-SR which is Pre-implantation Genetic Testing for Structural rearrangement, formally known as PGT for translocations.


What is the difference between a day 5, day 6 and day 7 embryo?

An embryo should make a blast around day 5 but sometimes they are a bit slow. These embryos may become blastocysts by day 6 and be suitable for freezing or transfer. If frozen, a day 5 would likely get chosen before a day 6 as it was not developmentally behind. Some labs now keep embryos until day 7 but once again, these are a bit slow and will likely get chosen last. Studies show that the chance of success with a day 6 is less than a day 5 but many babies still come from them. 


Is it possible to have frozen embryos PGS-tested after the fact?

Different labs have different protocols for how they do their embryo testing. Sometimes the embryos are biopsied (usually day 5) and then frozen. Other times if it is not possible to do fresh biopsy the embryos are frozen on day 3 and when multiple embryos are available they are thawed out, grown to day 5, and then biopsied, and this way the embryo can recover from the freeze/thaw for a couple of days before biopsy. But day 5 frozen embryos may still be biopsied if the lab allows for it, although there is a chance that the embryo is still too contacted that day to be biopsied and need to be left until day 6. Check with your lab what they prefer for their testing procedures.


What about an embryo would make it good for a fresh transfer but not to freeze?

Generally clinics have criteria for what is suitable for transfer or freeze. Every now and then we get embryos that are ok for transfer but not for freeze. This may be because there may only be poor embryos available and a doctor may choose to transfer one rather than cancel a transfer and it may not be suitable for freezing as we need to assess whether it can survive the freeze and thaw.


How come a perfectly graded embryo can fail to implant?

A perfectly graded embryo can be genetically abnormal. But even a genetically tested embryo that is perfectly graded is still not guaranteed to work. There may be issues with the embryo not detected in the testing, issues with the endometrium, hormones, fibroids/adhesions etc in the uterus among many other factors.


What is one misconception about embryos that you wish more patients understood about embryology or embryos?

I wish patients knew that the amount of eggs they get collected is not the number of embryos or tries for a baby that they will get. Some people are upset that the numbers drop off so dramatically when it’s actually normal and expected to happen. Getting 8 eggs collected doesn’t mean 1 for transfer and 7 embryos in the freezer. In reality, you would be doing really well if 6 fertilize and you end up with 1 for transfer and 2 in the freezer. 

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