Embryo grading is a scoring system that embryologists use during IVF to assess how an embryo looks under a microscope. It evaluates physical features like cell count, symmetry, and structure to estimate which embryos have the best chance of leading to a pregnancy. The grade is not a diagnosis or a guarantee. It’s a visual snapshot that helps your fertility team decide which embryo to transfer first.
How Day 3 Embryos Are Graded
Embryos can be graded at two key stages. The first is around day 3 after egg retrieval, when the embryo is still a clump of dividing cells called a cleavage-stage embryo. At this point, embryologists look at two main things: how many cells there are and how much fragmentation is present.
A healthy day 3 embryo should have about 6 to 10 cells. Ideally, you want to see around 8 cells by the 72-hour mark. Embryos that are dividing too slowly or too quickly may score lower, because abnormal division rates can signal problems with development. Fragmentation refers to tiny cell fragments that break off during division. A small amount is normal, but embryos with more than 25% fragmentation have significantly lower implantation potential. The best-graded embryos have evenly sized cells with little to no fragmentation.
Different clinics use slightly different numbering systems for cleavage-stage embryos, which can make it confusing when you try to compare grades across clinics. Some use a 1-to-4 scale (with 1 being best), while others reverse it. Always ask your clinic what their scale means rather than assuming.
How Day 5 Blastocysts Are Graded
Most clinics now grow embryos to day 5 or 6, when they reach the blastocyst stage. Blastocyst grading is more standardized, using the Gardner system. This produces a grade like “4AA” or “3BB” that contains three pieces of information: a number and two letters.
The number (1 through 6) describes expansion, meaning how much the embryo has grown and how large the fluid-filled cavity inside it has become. A 1 is just starting to form that cavity, while a 5 or 6 is fully expanded or even beginning to hatch out of its outer shell. Higher numbers generally indicate more advanced development.
The first letter grades the inner cell mass (ICM), the cluster of cells that will eventually become the baby. An “A” means a tightly packed, well-defined group of cells. A “B” is slightly less organized, and a “C” means the cells are sparse or hard to distinguish.
The second letter grades the trophectoderm, the outer layer of cells that will become the placenta. The same A-through-C scale applies: “A” is a smooth, cohesive layer with many cells, while “C” has fewer cells and a more uneven appearance. With 6 possible expansion scores and 3 grades for each cell layer, the system produces 54 possible combinations.
Which Part of the Grade Matters Most
Not all parts of the blastocyst grade carry equal weight. A large study of nearly 3,000 single embryo transfers found that the trophectoderm grade (the second letter) was a better predictor of implantation than the inner cell mass grade. Embryos with an “A” trophectoderm had a 48.8% pregnancy rate per transfer, compared to 42.4% for “B” and just 25% for “C.” The miscarriage rate also climbed as trophectoderm quality dropped: 17.8% for grade A versus 31.4% for grade C.
The inner cell mass grade, by contrast, did not show a statistically significant effect on pregnancy or miscarriage rates in the same study. That doesn’t mean ICM quality is irrelevant, but if you’re trying to understand your embryo’s prospects, the trophectoderm letter is the one to pay closer attention to.
What the Grade Means for Live Birth Rates
Embryo grade does correlate with outcomes, but the numbers are more modest than many people expect. For women aged 25 to 32, the live birth rate per embryo was about 51% for “good” quality embryos, 39% for “fair,” and 25% for “poor.” That means a top-graded embryo still has roughly a coin-flip chance of resulting in a baby, and a lower-graded one still has a real shot.
Age plays a major role on top of grading. For women aged 40 to 44, those same categories dropped to 22%, 14%, and 8%. This is largely because older eggs are more likely to carry chromosomal abnormalities, which grading cannot detect.
Grading Doesn’t Reveal Chromosomal Health
This is one of the most important things to understand about embryo grading: a beautiful-looking embryo can still be chromosomally abnormal, and a mediocre-looking one can be perfectly normal. Research has shown that more than half of embryos with good morphological grades turn out to be aneuploid (carrying the wrong number of chromosomes) when tested.
Good-quality blastocysts do have higher rates of chromosomal normality than poor ones. In women under 35, about 63% of good-quality embryos were chromosomally normal compared to 32% of poor-quality ones. In women 35 and older, even good-quality embryos had a euploidy rate of only about 42%. This is why many clinics recommend preimplantation genetic testing (PGT-A) alongside morphological grading, especially for older patients. The two tools measure completely different things: grading assesses physical appearance, while genetic testing counts chromosomes.
Grading Is More Subjective Than It Seems
Embryo grading feels precise when you see a clean alphanumeric score on your chart, but it involves real human judgment. A study that had 26 embryologists grade the same 35 embryos found substantial disagreement between observers. Some embryologists differed from the expert reference standard by as much as two full grades, even though everyone was using the same grading system. There was even moderate variability when the same embryologist graded the same embryo a second time. Clinics that perform a higher volume of IVF cycles tended to show less variability, likely because their teams get more practice calibrating with each other.
This means you shouldn’t treat the difference between, say, a 4AB and a 4BB as a dramatic gap. A small difference in grade could easily be a matter of who was looking at the embryo and when.
How Freezing Affects Embryo Grades
If your embryos are frozen (vitrified) and later thawed for transfer, their grades can change. A large study of over 7,700 frozen embryo transfers found that about 22% of embryos received a lower score after thawing, while a smaller group actually looked better. A small percentage (around 2.7%) did not re-expand at all after thawing.
These changes matter. Embryos that were downgraded after thawing had about 30% lower odds of live birth compared to those whose grade stayed the same. Embryos that failed to re-expand had even lower odds. On the other hand, embryos that improved after thawing had 42% higher odds of live birth than those that held steady. Your clinic will re-evaluate the embryo after warming and factor that into their assessment before transfer.
What to Do With Your Embryo Grades
When your clinic gives you a grade, it’s worth understanding what it means, but try not to fixate on it as a prediction. Grading is one tool among several. It helps embryologists rank your embryos relative to each other so they can transfer the most promising one first. It does not tell you whether a specific embryo will or won’t become a baby.
Plenty of lower-graded embryos result in healthy pregnancies, and plenty of top-graded embryos don’t implant. If you have multiple embryos, the grade helps with sequencing. If you have one embryo, the grade tells you something about probability but nothing about your individual outcome. The most useful question to ask your embryologist isn’t “Is this a good grade?” but rather “How does this embryo compare to the others we have, and why are you recommending this one for transfer?”