The best embryo grade in IVF is a fully expanded or hatching blastocyst scored AA, typically written as 4AA, 5AA, or 6AA. These embryos have the most cells, the tightest organization, and the highest chance of implanting. A 4AA euploid (chromosomally normal) embryo carries clinical pregnancy rates of 63 to 65% and live birth rates around 50% per transfer. But grading is only part of the picture, and lower-graded embryos still produce healthy pregnancies every day.
How Embryo Grading Works
Most clinics grade embryos that have reached the blastocyst stage, usually on day 5 or 6 after fertilization. The grading system uses a number followed by two letters. The number (1 through 6) describes how far the embryo has expanded. The first letter grades the inner cell mass, which becomes the baby. The second letter grades the trophectoderm, the outer ring of cells that becomes the placenta.
A score of 1 means a cavity is just beginning to form. By stage 3, the embryo is a full blastocyst. Stage 4 is expanded, stage 5 is hatching out of its shell, and stage 6 has fully hatched. Only embryos at stage 3 or beyond receive the two letter grades, because the inner cell mass and trophectoderm aren’t distinct enough to evaluate before that point.
Each letter runs from A (best) to C (poorest). An A for the inner cell mass means a large, tightly packed cluster of many cells. A B means a moderate number of loosely grouped cells. A C means very few cells, or no identifiable cell mass at all. For the trophectoderm, A means a continuous, uniform layer of many cells. B means fewer cells with some variation in size or shape. C means a sparse layer with gaps.
What Makes a Top-Grade Embryo
A 4AA embryo is the standard benchmark: fully expanded, with an excellent inner cell mass and excellent trophectoderm. Embryos graded 5AA or 6AA are similarly high quality but have progressed further in hatching. These top-grade blastocysts achieve live birth rates around 44% in large studies of single embryo transfers, climbing to roughly 50% when the embryo has also been confirmed chromosomally normal through genetic testing.
That said, the two letters don’t carry equal weight. Research on single-blastocyst transfers found that the trophectoderm grade was the strongest predictor of implantation and live birth, while the inner cell mass grade was not significantly associated with outcomes on its own. Live birth rates were 57% for trophectoderm grade A, 40% for grade B, and 25% for grade C. For the inner cell mass, grades A and B performed almost identically (53% vs. 52%), with only grade C showing a meaningful drop. So an embryo graded 4BA, for example, may perform nearly as well as a 4AA.
Day 3 vs. Day 5 Grading
Some clinics still transfer or assess embryos at day 3, before the blastocyst stage. At this earlier point, grading focuses on how many cells the embryo has and how much fragmentation is present. A top-quality day 3 embryo typically has 7 to 8 cells with less than 10% fragmentation, meaning the vast majority of the cell material is organized into intact, evenly sized cells rather than scattered fragments.
Day 3 grading is less predictive than blastocyst grading because many embryos that look good on day 3 stall before reaching blastocyst. Waiting until day 5 gives embryologists more information, since the embryo has already passed a critical developmental hurdle. Implantation rates for day 5 transfers (24.2%) are significantly higher than day 4 transfers (18.4%), reflecting the value of that extra development time. Embryos that haven’t reached at least the compaction or morula stage by day 5 have essentially no implantation potential.
Grade Doesn’t Guarantee Genetics
One of the most important things to understand about embryo grading is that it measures appearance, not chromosomal health. A study examining the relationship between morphology and genetic testing results found that roughly 52% of embryos with the best morphologic grade were chromosomally abnormal. Meanwhile, about 30% of poorly graded embryos turned out to be chromosomally normal.
This is why many clinics recommend preimplantation genetic testing, especially for patients over 35 or those with recurrent loss. A beautiful-looking 4AA embryo can carry an extra or missing chromosome, while a less impressive 3BB might be perfectly normal. When genetic testing confirms an embryo is euploid (has the correct number of chromosomes), success rates improve regardless of the morphology grade. The combination of good morphology and confirmed euploidy gives the highest probability of a live birth, but genetics matters more than appearance.
What Lower Grades Actually Mean for Your Chances
If your clinic tells you your best embryo is a 3BB or a 4BC, that doesn’t mean it won’t work. A large multinational study tracked outcomes across thousands of single blastocyst transfers and found live birth rates of 44.4% for good-grade embryos, 38.6% for moderate-grade, and 30.2% for low-grade. Even very low-grade blastocysts (CC) achieved a 13.7% live birth rate per transfer. Those odds are lower, but they’re real.
Crucially, that same study found no increase in adverse outcomes for babies born from lower-grade embryos. Preterm birth rates, birth weights, and rates of being small or large for gestational age were all comparable to babies born from top-grade embryos. A lower grade means a lower chance of the transfer succeeding, not a lower quality pregnancy or baby if it does.
Embryos graded with a C on only one component fared better than those with C on both. Blastocysts graded BC or CB had live birth rates around 24 to 33%, while CC embryos were closer to 14%. If your embryo has at least one A or B in the mix, your chances sit meaningfully higher than the lowest tier.
How Grade Affects Freezing and Thawing
If your embryos are being frozen for a later transfer, grade matters for survival. Good-quality blastocysts survive the freeze-thaw process about 90 to 95% of the time with modern vitrification techniques. Borderline or poor-quality blastocysts have lower survival rates, though most clinics don’t publish exact numbers for each grade. This is one reason embryologists may recommend against freezing the lowest-grade embryos: the chances of surviving the thaw and then implanting can become quite slim.
Your clinic will typically only freeze embryos they believe have reasonable potential. If an embryo isn’t frozen, it usually means the embryologist judged that it wouldn’t survive the process or wouldn’t have meaningful implantation potential, not that it was overlooked.
Putting the Numbers in Perspective
Embryo grading is a useful tool, but it’s a snapshot of how an embryo looks under a microscope at one moment in time. The grade your embryologist assigns is somewhat subjective, and there’s natural variation between clinics in how strictly they score. Two clinics might grade the same embryo differently.
Your age at the time of egg retrieval is the single most important factor in IVF success, more predictive than any embryo grade. A 4AA from a 40-year-old patient has a lower chance of being chromosomally normal than a 3BB from a 30-year-old. The grading system tells you about cell organization and development speed, which are meaningful, but they’re one piece of a larger picture that includes your age, uterine lining, hormone levels, and whether the embryo has been genetically tested.
If you have multiple embryos, your clinic will transfer the highest-graded one first. But if your only embryo is a 3BC, that embryo still has a real chance of becoming a healthy pregnancy. The grading system helps prioritize, not predict with certainty.