What Is Embryo Grading in IVF?

Embryo grading is a visual assessment performed by embryologists during IVF to evaluate embryo quality under a microscope at two stages. Day 3 cleavage embryos are graded by cell number and fragmentation with 6-10 cells being ideal. Day 5 blastocysts receive a number for expansion (1-6) and two letters for inner cell mass and trophectoderm quality (A to C). A grade like 5AA means a fully expanded blastocyst with excellent cell quality.

According to Dr. Hrishikesh Pai, renowned IVF Doctor in India, “Grades help us pick the best embryo for transfer but they’re not the whole story. B grade embryos make babies all the time and AA embryos sometimes don’t implant.”

Two different systems because the embryo looks completely different at each stage. Day 3 it’s a ball of cells. Day 5 it’s a blastocyst with two distinct parts and a fluid-filled cavity. The grading criteria change because the structure changes.

• Day 3 Cell Count: Ideal is 6-10 cells by day 3 with 8 cells being the target. Fewer than 6 means the embryo is dividing slowly. More than 10 and it may be dividing too fast. Cell number at this stage is the single strongest predictor of whether the embryo will make it to blastocyst
• Day 3 Fragmentation: Grade 1 has zero fragments and every cell is equal in size. Grade 2 has under 10% fragments which is still good. Grade 3 has 10-25% and Grade 4 has over 25%. More fragments mean less cellular material available for the embryo to develop properly through fertility treatment cycles
• Day 5 Expansion Number: Blastocysts get a number from 1 to 6. Grade 1 is early with a small cavity. Grade 3 is full blastocyst where the cavity fills the entire embryo. Grade 4 is expanded with thinning shell. Grade 5 is hatching. Grade 6 is fully hatched and ready for implantation
• Day 5 Letter Grades: Two letters follow the number. First letter grades the inner cell mass (becomes the baby) from A to C. Second letter grades the trophectoderm (becomes the placenta) from A to C. So 5AA means expanded blastocyst with excellent inner cell mass and excellent trophectoderm

Not every clinic uses the exact same grading scale but the logic is identical everywhere. More about the embryo culture process on the blastocyst culture page.

Here’s what matters. AA embryos have the highest implantation rates at 50-65% per transfer. BB embryos still sit at 40-50%. Even BC embryos produce healthy pregnancies. The grade is a guide not a verdict and it tells you nothing about what’s happening inside the embryo genetically which is why PGT-A exists as a separate test.

• Top Grade (AA): 5AA or 4AA blastocysts have 50-65% implantation rates per transfer. These have tightly packed inner cell mass and organized trophectoderm and are always the first choice for IVF treatment transfer when available
• Good Grade (BB): 4BB and 3BB embryos still achieve 40-50% implantation. Most IVF pregnancies actually come from B grade embryos because they’re more common than AA and the success rate difference isn’t as dramatic as patients expect
• Lower Grade (BC/CB): Pregnancy rates drop to 20-30% but babies born from C grade embryos are completely healthy. The grade reflects how the embryo looks on the outside not whether the chromosomes inside are normal or abnormal
• Grading Limitations: Two embryologists looking at the same embryo can assign different grades because the system is subjective and based on visual appearance only. An AA embryo can be aneuploid and a BB embryo can be perfectly euploid which is why combining grading with PGT-A gives the most complete picture

A lower grade doesn’t mean a failed cycle. It means slightly lower odds per transfer. Couples who want the clearest answer about chromosomal health should consider adding PGT-A. More about that in this egg freezing guide.

Dr. Hrishikesh Pai has been doing this for over 40 years. MD and FRCOG (UK-HON) and MSc (USA) and FCPS and FICOG. He started the Bloom IVF Group and the count is past 25,000 IVF cycles across eight centers. Lilavati Hospital Mumbai and DY Patil Navi Mumbai and Fortis in Delhi and Gurgaon and Mohali. His labs run Life Whisperer AI for picking the best embryos.

Hormonal workups and imaging and tubal checks happen before anything gets recommended. FIGO World Congress keynote speaker. BBC World Service feature on egg freezing.

References

1. Embryo grading and IVF outcomes – ARC Fertility
2. Blastocyst grading systems – National Library of Medicine