Blog
Understanding Embryo Grading and why it’s important
Alease Daniel
Senior Embryologist
Introduction
By far one of my most asked questions is about embryo grading. Breaking down that embryo report can be daunting and going down the Google rabbit hole only makes it confusing.
Hopefully, this guide can break it down in a way that makes sense, but keep in mind, not all labs grade this way!
If you are ever unsure of what something means, ASK, and if your clinic isn’t interested in answering your questions, that’s a big red flag in my book. You’re investing so much time, money, and emotional effort into the process, you deserve to understand what’s going on with your care.
Grading your Embryos
Let’s get down to it! Majority of labs are grading embryos based on SART guidelines. SART stands for The Society for Assisted Reproductive Technology. Labs are not required to follow SART guidelines for grading, but it was created to try to standardize grading across clinics. SART grades day 5,6 and 7 embryos based on three things: expansion, ICM and trophectoderm. This leads to grades that look something like this: 3AB, 4BB, 2BC, etc.
Expansion: This is the number in the grading system. Numbers go from 1-6 and tell you how mature, or how thin the zona of the embryo is. The zona is the shell of the embryo. As the cells of the embryo continue to multiply and grow, the zona thins and eventually the embryo hatches out of the zona. All embryos have to hatch to implant into the uterus! Expansion doesn’t necessarily indicate the quality of the embryo, but it does tell you its maturity. Expansion numbers 1-4 are embryos that have not yet hatched out of the zona. 1 and 2 are very early in development and often are not ready for freezing, transfer, or biopsy depending on the lab. Numbers 3 and 4 are the most common to see and indicate a good level of maturity. Number 5 indicates when an embryo is hatching out of the embryo. This one can be a little misleading, because some labs artificially hatch, or laser, their embryos on day 3. This will mean that most of the embryos will be hatching on day 5, but not naturally. Hatching artificially is not a bad thing, just something to keep in mind. The last expansion grade is 6. This is when the embryo is fully hatched out of the zona. These embryos have slightly higher implantation rates but are actually more difficult for embryologists to work with and are more likely to have cryo damage.
Inner cell mass or ICM: This is the first letter in the grading system. The inner cell mass is what becomes the baby and is basically a small grouping of cells inside the embryo. The letters for grading the ICM go from “A” to “D” with most clinics not freezing any embryos past a “C” grade ICM. The letter “A” is considered good, “B” is fair and “C” is poor. Technically based on SART guidelines, a “C” is an embryo with no visible ICM, but this is one where it very much depends on the clinic. A good ICM is well organized, compact and has little cell death.
Trophectoderm: This is the last letter in the grading system. The trophectoderm is what eventually becomes the placenta. It is graded exactly like the ICM, using letters “A” to “D”. When looking at a good trophectoderm, it will have a large number of cells and little to no “bald” spots. There will be very few extruded or dead cells. The trophectoderm becomes even more important when a patient is doing biopsy for PGT testing. Embryos with poor quality trophectoderms are not good candidates for biopsy because we have to be able to take 6 to 8 cells while still leaving enough cells for the embryos to survive. It is unlikely that “C” graded embryos will have enough cells for an embryologist to feel comfortable biopsying it. It is much safer to just freeze “C” grades as no cells are being taken from the embryo.
Now that we’ve broken down each part of the grading system, we can put all three parts together. SART also has guidelines on what embryos are good, fair and poor based on the combination of ICM and trophectoderm grades. Expansion is not included in this part of the grading, as remember, it doesn’t necessarily mean an embryo is good or bad. SART breaks down “good” embryos as those that have “AA” or “AB” grades. “Fair” is considered embryos that have the grades “BA” and “BB” and “poor” are any grades with a “C” (BC, CB, CC).
Conclusion
A few final thoughts about grading. Hatching embryos often look like they’re splitting, but they’re not! We can not tell if an embryo is going to split into identical twins and there is nothing we can do to cause it. We also can’t tell just by looking at an embryo, if it is male or female. The only way to do that is embryo biopsy for PGTA testing. While grading is important, it doesn’t mean that your poorly graded embryo won’t make a beautiful healthy baby!