Decode your embryo's grade, see what each number and letter means, and what the largest studies found grades do and do not predict. Lower grades become healthy babies every day.
An embryo grade like 4AA is three judgements from the Gardner scale most labs use: the number (1 to 6) is how far the blastocyst has expanded, the first letter grades the inner cell mass that becomes the baby, and the second grades the trophectoderm that becomes the placenta. Higher grades lead to more live births on average, 44.4% for good-grade blastocysts against 30.2% for low-grade ones in the largest study, but a grade reads how an embryo looks, not its chromosomes.
Type the grade your clinic gave you, or pick its three parts. Each part is explained, with what the largest study found for grades like it.
Type it as your clinic wrote it, or pick the parts below.
Expansion (the number)
Inner cell mass
Trophectoderm
An expanded blastocyst (4) with an A inner cell mass and an A trophectoderm.
The cavity has grown larger than the original embryo and the shell around it (the zona) is thinning.
Many cells, tightly packed.
Many cells forming a smooth, continuous layer.
In the largest study of grades and births (Zou 2023: 10,964 single blastocyst transfers, mostly frozen, average age 33), 44.4% of good-grade blastocysts (AA, AB, BA) led to a live birth. That is an average across thousands of patients, not a prediction for yours.
A blastocyst is graded on day 5 or 6. The number comes first; the two letters follow from expansion 3 upward, once the cell groups can be judged.
A fluid-filled cavity has formed but fills less than half of the embryo.
The cavity fills half or more of the embryo.
The cavity fills the whole embryo.
The cavity has grown larger than the original embryo and the shell around it (the zona) is thinning.
The outer cells have started to push out through the shell.
The embryo has completely escaped its shell.
The cells that become the baby.
Many cells, tightly packed.
Several cells, loosely grouped.
Very few cells.
The outer layer that becomes the placenta.
Many cells forming a smooth, continuous layer.
Few cells forming a loose layer.
Very few, large cells.
The scale is Gardner and Schoolcraft's (1999), in the definitions printed in Gardner 2000. Grading is a trained judgement made through a microscope, labs apply it with some variation, and a few use other scales, so your lab can always tell you what its grades mean.
Want it on one page? Save the free chart: how to read an embryo grade, with live birth by grade, to pin, download or share.
The largest study linking grades to births followed 10,964 single blastocyst transfers at 14 clinics in Australia, China and New Zealand, 2009 to 2020. Most (79.9%) were frozen transfers; the average age was 33.
| Group (the study's name) | Letters | Transfers | Live birth |
|---|---|---|---|
| Good grade | AA, AB, BA | 4,386 | 44.4% |
| Moderate grade | BB | 3,735 | 38.6% |
| Low grade | AC, CA, BC, CB, CC | 2,843 | 30.2% |
| Very low grade | CC | part of low | 13.7% |
Inside the low-grade group the pairs differ: AC, CA and BC each led to a live birth about 33% of the time, CB 24.6% and CC 13.7%. In the same study, low-grade blastocysts did not carry a higher rate of pregnancy loss than good-grade ones, and the babies born from them had comparable preterm-birth rates and birth weights.
These are averages across thousands of patients. Your own odds depend on far more than the grade, above all your age (see IVF success rates by age), so a grade is best read as one input your clinic weighs when choosing which embryo to transfer first.
Source: Zou H, Kemper JM, Hammond ER, et al. Blastocyst quality and reproductive and perinatal outcomes: a multinational multicentre observational study. Hum Reprod 2023;38(12):2391-2399. Blastocysts at expansion 1 or 2 were not included.
Every large study found the second letter, the trophectoderm, matters. Older single-country studies found the first letter, the inner cell mass, added little once the trophectoderm was known; the largest recent study found both letters mattered on their own, and a C in the inner cell mass lowered the live-birth rate the most.
10,964 single blastocyst transfers at 14 clinics in Australia, China and New Zealand, 2009 to 2020
Both letters mattered on their own. Live birth was 44.8%, 37.2% and 23.2% for an A, B and C inner cell mass, and 45.5%, 37.7% and 32.0% for an A, B and C trophectoderm.
3,151 fresh, day 5 elective single blastocyst transfers reported to SART (US national data), 2008 to 2009
Trophectoderm quality and expansion each predicted live birth independently; inner cell mass did not. About half of transfers with a good trophectoderm led to a live birth (50%), against 41.9% with a fair one. This study used SART's good, fair and poor scale rather than Gardner's letters.
694 single blastocyst transfers in fresh cycles at one large US practice
Live birth was 57%, 40% and 25% for a trophectoderm graded A, B and C. Only age and trophectoderm grade were significantly tied to live birth; inner cell mass grade was not.
1,117 fresh, day 5 single blastocyst transfers at one Swedish clinic
All three parts of the grade were linked to live birth, but after adjusting for other factors the trophectoderm was the only independent predictor.
Grading reads how an embryo looks; PGT-A genetic testing reads its chromosomes. They answer different questions.
A grade judges how an embryo looks under a microscope, not its chromosomes. A top-graded embryo can be chromosomally abnormal and a lower-graded one can be normal and become a healthy baby, which is why genetic testing (PGT-A) is a separate step.
Better grades are more often normal, not always. In Capalbo 2014 (956 blastocysts tested at two centers), 56.4% of top-grade blastocysts (3AA or better) were chromosomally normal, against 25.5% of poor-grade ones: so many top-grade embryos are not normal, and about a quarter of poor-grade ones are.
Grade still mattered among embryos that tested normal. In Irani 2018 (701 frozen single transfers of euploid blastocysts at one New York center), live birth was 67.8% for good-grade embryos (AA, AB, BA), 53.4% for BB and 29.5% for BC or CB. See what PGT-A costs and what the trials found.
Some embryos reach the blastocyst stage on day 5, others a day later. The day is recorded alongside the grade.
On average, day 5 blastocysts lead to more live births. A meta-analysis of 13 studies (Bourdon 2019) found a live birth 1.50 times as likely with a day 5 blastocyst overall (95% CI 1.32 to 1.69), 1.74 times in fresh transfers and 1.38 times in transfers of vitrified embryos. None of the studies was a randomized trial.
For genetically tested (euploid) embryos the meta-analysis found no significant difference, but only two studies could be pooled for that question. In Irani 2018's genetically normal embryos, day 5 transfers led to a live birth 60.4% of the time and day 6 transfers 44.8%: lower, and still a real chance.
Before the blastocyst stage, an embryo is described by its cell count and a quality grade.
An optimal day 3 embryo has 8 cells of equal size, each with a single nucleus, and less than 10% fragmentation (small pieces of cell material that have broken off).
Good. Under 10% fragmentation, cells the size expected for their stage, no cell with more than one nucleus.
Fair. 10 to 25% fragmentation, most cells the expected size, no sign of extra nuclei.
Poor. Over 25% fragmentation, cells not the expected size, or a cell with more than one nucleus.
Expected cell count: 4 on day 2, 8 on day 3. Source: Istanbul consensus 2011.
Fewer embryos reach day 5 than were fertilized, and that is expected. Embryologists' own lab standard expects at least 40% of normally fertilized eggs to become blastocysts by day 5, and 60% or more in the best labs, with another 10 to 15% by day 6.
These are targets for a lab across many patients, not a forecast for your cycle. Source: Vienna consensus 2017.
It is the Gardner scale, three judgements in one. The number (1 to 6) is how far the blastocyst has expanded: 4 is an expanded blastocyst whose shell is thinning. The first letter (A to C) grades the inner cell mass, the cells that become the baby, and the second letter grades the trophectoderm, the cells that become the placenta; A is the most cells in the best arrangement. So 4AA is an expanded blastocyst with top-graded cells in both groups. The letters are given only from expansion 3 upward.
In the largest study of grades and births (Zou 2023, 10,964 single blastocyst transfers), "good grade" meant AA, AB or BA at expansion 3 or higher, and 44.4% of those transfers led to a live birth. BB, called moderate, led to a live birth 38.6% of the time, and any grade with a C 30.2%. Those are averages across thousands of patients, mostly frozen transfers at an average age of 33, not a prediction for one embryo.
Yes. In Zou 2023, 30.2% of low-grade blastocysts (any grade with a C) led to a live birth, about 33% for AC, CA and BC, 24.6% for CB and 13.7% for CC. In the same study, low-grade blastocysts did not carry a higher rate of pregnancy loss than good-grade ones, and the babies born from them had comparable preterm-birth rates and birth weights.
Every large study found the second letter, the trophectoderm, matters. Older single-country studies found the first letter, the inner cell mass, added little once the trophectoderm was known; the largest recent study found both letters mattered on their own, and a C in the inner cell mass lowered the live-birth rate the most. In Zou 2023, live birth fell from 44.8% to 23.2% between an A and a C inner cell mass, and from 45.5% to 32% between an A and a C trophectoderm.
On average it leads to fewer live births. A meta-analysis of 13 studies (Bourdon 2019) found live birth 1.50 times as likely with a day 5 blastocyst overall, 1.74 times in fresh transfers and 1.38 times in transfers of vitrified (frozen) embryos. For genetically tested (euploid) embryos the meta-analysis found no significant difference, but only two studies could be pooled for that question. A day 6 blastocyst is still a real chance: in a study of 701 single transfers of genetically normal embryos (Irani 2018), 44.8% of day 6 transfers led to a live birth, against 60.4% for day 5.
Yes, it still made a difference. In Irani 2018, 701 frozen single transfers of euploid (chromosomally normal) blastocysts led to a live birth 67.8% of the time for good-grade embryos (AA, AB, BA), 53.4% for BB and 29.5% for BC or CB. A grade and a genetic test measure different things, and both carry information.
By the international consensus on embryo assessment (Istanbul consensus 2011), the expected embryo has 4 cells on day 2 and 8 cells on day 3. An optimal day 3 embryo has 8 cells of equal size, each with a single nucleus, and less than 10% fragmentation (small pieces of cell material that have broken off). Its grade 1 (good) means under 10% fragmentation, grade 2 (fair) 10 to 25%, and grade 3 (poor) over 25%. An embryo that keeps growing is graded again as a blastocyst on day 5 or 6.
Not all of them, and that is normal. The lab performance standard set by embryologists (Vienna consensus 2017) expects a competent lab to see at least 40% of normally fertilized eggs become blastocysts by day 5, and the best labs 60% or more, with another 10 to 15% forming by day 6. These are targets for a lab across many patients, not a prediction for one cycle.
Where grading happens in a cycle, from stimulation to the pregnancy test, and how long each step takes.
Most graded blastocysts are frozen first. What a frozen transfer costs, its success and its timeline.
The CDC's national live-birth rates by age, the factor that moves the odds most.