Each embryo starts as a single cell. That single cell will divide every 12 to 24 hours and by the time an embryo has reached day 3 of growth, it should be between four and eight cells. Day 3 and day 5 of embryo growth are only 2 days apart, but there is a significant difference between these two stages of maturation. Embryos grow rapidly.
Exactly what are day 3 embryos? At this stage, the cells inside the embryos are dividing — embryologists call this the “cleavage” stage — but they aren’t growing in size. The embryos are still at the same size as an unfertilized egg.
Just two days later, on day 5, an embryo will consist of between 70 and 100 cells. At this stage of growth, the embryo is referred to as a blastocyst. A blastocyst consists of two types of cells, those that will develop into fetal tissues, and those that will develop into the placenta. Only about one-third of embryos successfully reach the blastocyst stage of development.
Day 3 embryo grading system
At day 3, embryologists use a high-power microscope to take a look at the morphology (a fancy word for “structure”) of the embryo. They’re looking mainly at two things:
- the number of cells in the embryo: An embryo that’s dividing well should ideally have between 6 to 10 cells by day 3. Research Trusted Source shows that 8 is best. (Day 3 embryos that had 8 or more cells showed a significantly higher live birth rate). However, it does seem that the number of cells in an embryo is the best indicator of whether an embryo will thrive or not.
- what they look like: While it’s relatively easy to count the number of cells you see in a day 3 embryo, cell appearance is harder to grade. This is especially so because sometimes you aren’t looking at the embryo head-on, but at a tangent. Day 3 embryos are graded 1 to 4 (or 5) depending on the lab protocol with 1 being the highest grade.
So, what are embryologists looking for?
- They want to see that each cell has a nucleus and that the cells are of equal size.
- They also check for fragmentation. This sometimes occurs when cells divide. Think of the crumbs falling off that birthday cake. Up to 20 percent fragmentation is fine. More than that and the cells lose too much cytoplasm (cell contents) for optimal function.
Other things that fertility specialists will take into account when examining a 3-day embryo are:
- Compaction — are the cells compacted and readying for the next stage, which is forming a blastocyst?
- Cytoplasmic pitting – are there depressions in the cell contents? Research is divided about what this means.
- Vacuoles -are there fluid-filled pockets?
Day 5 embryos grading system
Some fertility clinics prefer to transfer embryos once they reach day 5. Your embryo (now called a blastocyst) is hard at work dividing and building up the number of cells. Here’s where division of labor comes in:
- Some of the cells form the inner cell mass (ICM). These develop into the fetus.
- Some of the cells form the trophectoderm epithelium (TE). These develop into the placenta and other tissues that your body needs for the pregnancy.
At this point, things are getting pretty cramped inside the shell (zona pellucida or ZP for short) that surrounds the embryo — and the shell is being pushed to its limits. As the embryo readies to burst through and implant itself in the lining of your uterus, the membrane of the ZP shell start to thin out.
If you thought that grading a day 3 embryo was challenging, things get even more complicated with day 5 embryos. Although grading is more or less universal, every fertility center has a unique system and that accounts for the slight differences that you may see in grades.
What’s being graded this time round?
- The amount that the blastocyst has expanded. The part of the embryo that’s being measured here is the blastocoel. The blastocoel is the fluid-filled cavity that forms within the embryo. Grades range from 1 to 6, with 6 being the most developed.
- The quality of the ICM. Remember, the ICM forms the fetus. Here, grades range from A to C, with A being the best quality.
- The quality of the TE. The TE forms the placenta and other pregnancy-related tissues. Grades range from A to C, with A being the best quality.
So, as an example, you may have a day 5 embryo that’s graded as 5AA. This would be described as a hatching blastocyst with the highest quality ICM cells (first letter) and the highest quality TE cells (second letter).
Sources:
- https://www.healthline.com/health/infertility/embryo-grading#takeaway
- https://www.pfcla.com/blog/day-3-vs-day-5-embryo-transfer
By Dina Jaoshvili
