Embryo Grading: What the Numbers and Letters Mean

The lab called or emailed and gave you a list ("4AA, 3BB, 4BB, 3BC, 5BA") and you are scanning forums to figure out what the numbers and letters mean. You want to know if your 3BB is "as good as" your friend's 4AA, whether you should ask for the highest grade transferred first, and whether grading even matters once PGT-A is in the picture.

Embryo grading is morphology: a thirty-second visual assessment of expansion, inner cell mass, and trophectoderm under the microscope. The most common grading system in US labs is the Gardner scale, which produces those three-character codes like 4AA or 3BB¹. Once you know what the number and letters describe, you can read the report without inflating or deflating the meaning of the grades.

Grading is morphology, not destiny. I have watched a 3BB become a baby and a 5AA fail to implant. The report orders the embryos by probability. It does not predict which one becomes your pregnancy. This post explains what grading measures, the Gardner scale in detail, how day of grading changes the picture, what grading does and does not predict, and how it interacts with PGT-A ploidy for transfer selection.

What does embryo grading actually measure?

Grading is a visual assessment of blastocyst morphology at day 5, 6, or sometimes day 7 of culture. The embryologist looks at the embryo under a microscope and assigns a code based on three features:

1. How expanded the blastocyst is (a number)
2. The quality of the inner cell mass (ICM), the dense cluster of cells that will become the fetus (a letter)
3. The quality of the trophectoderm (TE), the outer layer of cells that will become the placenta (a letter)

The assessment takes thirty to sixty seconds per embryo. It is subjective enough that two embryologists grading the same embryo can disagree by a step, with reported inter-rater variability around twenty to thirty percent in published comparisons. That subjectivity is one reason your clinic's grade is not a verdict.

The Istanbul consensus published in Human Reproduction in 2011 attempted to standardize morphology assessment across labs, but variation persists in practice². Different clinics may use slightly different scales (some use 1 to 5 for expansion rather than 1 to 6; some use AA through DD), so always ask your clinic which system they use before comparing grades across reports.

How does the Gardner scale work?

The Gardner system is the most widely used in US clinics. Each blastocyst gets three components.

Expansion stage: a number from 1 to 6

The number reflects how much the blastocyst has expanded. As an embryo develops past the morula stage, a fluid-filled cavity called the blastocoel forms and grows. The number captures how big that cavity has gotten relative to the embryo and whether the embryo has begun to hatch out of its outer shell, the zona pellucida.

• 1: early blastocyst, blastocoel takes up less than half of the embryo volume
• 2: blastocyst, blastocoel fills more than half of the embryo volume
• 3: full blastocyst, blastocoel fills the entire embryo
• 4: expanded blastocyst, the embryo is larger than its original size and the zona is thinning
• 5: hatching blastocyst, the embryo is starting to push through the zona
• 6: fully hatched blastocyst, the embryo has exited the zona

Higher numbers do not necessarily mean better outcomes. They mean the embryo is further along in development at the moment of grading. A day 5 blastocyst graded 3 may catch up to a 4 by morning. A day 6 blastocyst graded 6 has hatched on schedule for day 6.

Inner cell mass (ICM): a letter, A or B or C

The ICM letter describes the cluster of cells that will become the fetus.

• A: tightly packed, many cells
• B: loosely grouped, several cells
• C: very few cells

ICM quality is one of the stronger predictors of clinical pregnancy in the Gardner literature, particularly for untested embryos.

Trophectoderm (TE): a letter, A or B or C

The TE letter describes the outer epithelium that becomes placenta.

• A: many cells forming a cohesive epithelium
• B: few cells, loose epithelium
• C: very few large cells

Trophectoderm grade also predicts implantation. Hill and colleagues published a 2013 study in Fertility and Sterility showing TE grade independently predicts single-blastocyst transfer outcomes⁴.

Putting it together

A 4AA blastocyst is an expanded blastocyst with top-grade ICM and top-grade TE. A 3BB is a full blastocyst with mid-grade ICM and TE. A 5BC is a hatching blastocyst with mid-grade ICM and lower-grade TE. The combination of number plus two letters is your embryo grading chart vocabulary, and once you have it, every report reads cleanly.

Why does the day of grading matter?

A 4AA on day 5 is not the same as a 4AA on day 7, even though the morphology code is identical.

• Day 5 blastocysts are standard timing. Per-transfer live birth rates are the reference point for the Gardner data¹.
• Day 6 blastocysts have developed more slowly. Per-transfer live birth rates are typically slightly lower than day 5 at matched grade, but still meaningful. Most clinics freeze and transfer day 6 blasts as standard practice.
• Day 7 blastocysts are the slowest-growing. Older protocols discarded them. Recent data, including the 2019 Human Reproduction analysis by Tiegs and colleagues, show that day 7 blastocysts have lower euploidy rates but, when euploid and transferred, produce sustained implantation rates similar to day 5 and day 6 embryos⁶. Some clinics now routinely freeze and transfer day 7s; others still discard.

When you read your report, the day matters as much as the grade. Ask your embryologist which day each embryo was graded.

What does embryo grading predict, and what does it not?

Grading predicts implantation rate. The Gardner data and subsequent studies are consistent: higher grades implant more often than lower grades, all else being equal.

Rough ranges for untested single blastocyst frozen embryo transfers in patients under 38:

• 4AA, 5AA, top-cluster grades: roughly 55 to 65 percent implantation per transfer
• 3BB, 4BB, mid-cluster grades: roughly 40 to 50 percent
• 3BC, 4CC, lower-cluster grades: roughly 25 to 35 percent

These numbers shift with maternal age, clinic protocols, and individual factors. Treat them as orientation, not precision.

Grading predicts euploid rate weakly. The 2014 Human Reproduction study by Capalbo and colleagues looked at the correlation between standard blastocyst morphology and euploidy across 956 screened blastocysts³. Top-grade embryos (AA-AA) had slightly higher euploid rates than lower-grade embryos (CC-CC), but the overlap was substantial. You cannot infer ploidy from morphology with any reliability.

Grading does not strongly predict live birth in PGT-A tested cycles. Once an embryo is confirmed euploid, grade matters less. A euploid 3BB and a euploid 4AA have similar live birth rates per transfer in most published cohorts.

Grading does not predict birth outcomes, child development, or anything past implantation. There is no published evidence that grade influences pregnancy course or child health among live births.

Grading or PGT-A: which matters more?

This is the most useful thing this post can teach, so I want to be direct.

For PGT-A tested cycles, ploidy is the primary selector and grade is the tiebreaker. If you have two euploid embryos, transfer order is decided by grade. If you have one euploid 3BB and one aneuploid 5AA, the 3BB wins by orders of magnitude. A euploid 3BB has a real shot at a baby. An aneuploid 5AA does not.

For untested cycles, grade is the primary selector. The embryologist will order your embryos by Gardner grade, with day of grading factored in, and the team will recommend transferring the top-cluster grade first.

For mosaic results, grade interacts with mosaic level in some clinic algorithms. A higher-grade low-level mosaic may be transferred ahead of a lower-grade mosaic in the same category. The Viotti embryo ranking system from the 2021 F&S paper formalizes this for clinics that want a structured framework⁵.

Can a lower-grade embryo still become the baby?

I see this in clinic often enough that I want to flag it. Live births from 3CC and 4CC embryos are documented in registry data and in my own practice. Some clinics will transfer "lower" grade embryos in untested cycles where the top-cluster grades did not implant, and those transfers do work.

Grading is a probability tool, not a verdict. The phrase "good embryo" and "bad embryo" is a clinical shorthand that hides the real picture, which is a continuous distribution of implantation odds with overlap at every grade level. If your report has a 3BC and a 4AA, the 4AA is the first transfer. If the 4AA fails, the 3BC is not the failure. It is the next attempt.

Should you freeze day 6 and day 7 blastocysts?

Patients often ask whether a day 6 or day 7 embryo is "worth" freezing. The honest answer is usually yes.

Day 6 blastocysts are standard practice to freeze and transfer. Per-transfer live birth rates are slightly lower than day 5 at matched grade, but the difference is small and the embryo is still worth banking.

Day 7 blastocysts are clinic-policy dependent. The Tiegs 2019 data and follow-on studies support transfer in the 15 to 25 percent live birth range per transfer when euploid⁶. Some clinics freeze them routinely; others discard. Ask your clinic's day 7 policy in advance, before retrieval, so you are not surprised by the report.

How does cleavage stage (day 3) grading work?

Some clinics still grade and report day 3 embryos, particularly for limited cohorts that may not reach blastocyst stage. Day 3 grading is by cell number (the target is six to eight cells on day 3) and degree of fragmentation (the lower the better).

Day 5 transfer outpaces day 3 transfer for live birth in most published populations, and the ASRM/SART committee opinion on blastocyst culture supports blastocyst transfer as standard practice⁵. If your clinic is recommending day 3 transfer, ask why specifically and what the alternative would be.

What should you ask your embryologist?

A short list when you get the grading report.

• What grading system do you use? Gardner or a clinic-specific variant?
• What is the day of grading for each embryo on my report?
• Which embryo would you transfer first based on grade plus ploidy if PGT-A was done?
• Are any embryos still being watched to see if they expand by day 6 or day 7?
• What is your clinic's live birth rate by grade for my age group?
• What is your clinic's day 7 freeze and transfer policy?

These are answerable questions. A clinic that gives you specific numbers is doing the work.

What does embryo grading mean for your transfer plan?

The embryo grading report is a probability ordering. Use it as that. If you have multiple euploid embryos, transfer order is grade-based. If you have untested embryos, transfer order is also grade-based. If your report has only lower-grade options, the right reading is "these are the embryos we have, and lower grades produce live births at lower but real rates."

What embryo grading does not tell you is which one will be the pregnancy. That part is biology and luck, in a proportion no clinic and no test can name precisely.

What's next

• If you have euploid embryos plus a mosaic and are deciding transfer order: should-transfer-mosaic-embryo
• If your PGT-A report came back and you need to read it row by row: pgt-a-results-meaning
• If you want the broader PGT testing context: pgt-a-m-sr-explained
• If a transfer has already failed and you are revisiting transfer order: failed-ivf-decoding-next

Sources

1. Gardner DK, Lane M, Stevens J, Schlenker T, Schoolcraft WB. Blastocyst score affects implantation and pregnancy outcome: towards a single blastocyst transfer. Fertility and Sterility 2000;73(6):1155-1158. https://doi.org/10.1016/s0015-0282(00)00518-5
2. Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology. The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting. Human Reproduction 2011;26(6):1270-1283. https://doi.org/10.1093/humrep/der037
3. Capalbo A, Rienzi L, Cimadomo D, et al. Correlation between standard blastocyst morphology, euploidy and implantation: an observational study in two centers involving 956 screened blastocysts. Human Reproduction 2014;29(6):1173-1181. https://doi.org/10.1093/humrep/deu033
4. Hill MJ, Richter KS, Heitmann RJ, et al. Trophectoderm grade predicts outcomes of single-blastocyst transfers. Fertility and Sterility 2013;99(5):1283-1289.e1. https://doi.org/10.1016/j.fertnstert.2012.12.003
5. Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. Blastocyst culture and transfer in clinically assisted reproduction: a committee opinion. Fertility and Sterility 2018;110(7):1246-1252. https://doi.org/10.1016/j.fertnstert.2018.09.011
6. Tiegs AW, Sun L, Patounakis G, Scott RT. Worth the wait? Day 7 blastocysts have lower euploidy rates but similar sustained implantation rates as Day 5 and Day 6 blastocysts. Human Reproduction 2019;34(9):1632-1639. https://doi.org/10.1093/humrep/dez138