As an embryologist, by far one of my most frequently asked questions is about IVF embryo grading. Breaking down that embryo report can be daunting, and going down the Google rabbit hole only makes it more confusing.
Hopefully, this guide can break it down in a way that makes sense, but please keep in mind, not all labs grade embryos this way! If you are ever unsure about what something means, ask. And if your clinic isn’t interested in answering your questions, that’s a big red flag. You’re investing so much time, money, and emotional energy into the process, you deserve to understand what’s going on with your care.
Now let’s get down to it! The majority of labs grade embryos based on SART guidelines (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 days 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 is represented by the number in the embryo grading system. Numbers go from 1-6 and tell you how mature or thin the zona, or shell of the embryo, is. 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 means 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 goes 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.
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 a biopsy for PGT testing. Embryos with a poor quality trophectoderm 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 embryo to survive. It is unlikely that “C” graded embryos will have enough cells for an embryologist to feel comfortable biopsying them. 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 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).
A few final thoughts about embryo grading
Hatching embryos often look like they’re splitting, but they’re not. We cannot 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 PGT-A testing.
The last thing to note is that while embryo grading is important, it doesn’t necessarily mean that your poorly graded embryo won’t make a beautiful healthy baby!
Alease Daniel is an Embryologist and experienced IVF Laboratory Technician residing in Raleigh, North Carolina. In her free time, she enjoys baking and cooking with her husband and playing with her two pups! You can follow Alease on Instagram at @alease_the_embryologist or TikTok at @aleasetheembryologist.