IVF blastocyst grading uses three components: a number (1–6) for expansion stage, and two letters (A–C) for inner cell mass and trophectoderm quality. A 5AA is the highest grade; 3CC is the lowest typically frozen. But grading is subjective and captures morphology, not genetics — even C-grade embryos result in healthy pregnancies, and PGT-A normal embryos have strong outcomes regardless of grade.
Key Takeaways
Blastocyst grades have 3 parts: expansion number (1-6), inner cell mass letter (A-C), trophectoderm letter (A-C) — 5AA is top, 3CC is lowest commonly frozen
AA embryos implant ~55-65% of the time; BC embryos still implant 15-25% — lower-grade embryos are absolutely worth transferring
Grading is subjective — two embryologists may grade the same embryo differently. Don't compare grades across clinics.
PGT-A (genetic testing) is a stronger predictor than morphological grade. An euploid 4BB has strong implantation odds.
What Those Letters and Numbers Actually Mean
Your clinic says your embryo is a "5AA" or a "4BB" or a "3BC" — and you immediately spiral into Googling what that means and whether your embryo is "good enough." You're not alone. Embryo grading is one of the most anxiety-inducing parts of IVF, partly because the grading system is confusing and partly because it's treated as more predictive than it actually is.
Here's what the grades mean, what they predict, and what they don't.
Day 5–6 Blastocyst Grading: The Standard System
Most clinics use the Gardner grading system for blastocysts (day 5–6 embryos). The grade has three components:
| Component | What It Measures | Scale | Best Score |
|---|---|---|---|
| Number (1–6) | Blastocyst expansion/development stage | 1 (early) → 6 (hatched) | 5 or 6 |
| First letter (A–C) | Inner cell mass (ICM) quality — becomes the baby | A (many cells, tightly packed) → C (few cells) | A |
| Second letter (A–C) | Trophectoderm (TE) quality — becomes the placenta | A (many cells, cohesive) → C (few cells, fragmented) | A |
So a 5AA means: fully expanded blastocyst (5) with an excellent inner cell mass (A) and excellent trophectoderm (A). A 3BC means: full blastocyst (3) with a poor inner cell mass (B) and few trophectoderm cells (C).
💡 Grading is subjective. Two embryologists can grade the same embryo differently. A '4AB' at one clinic might be a '4BA' at another. Don't compare grades across clinics — compare within your own cohort.
What Grades Predict (and Don't)
| Grade | Implantation Rate (approx.) | Clinical Interpretation |
|---|---|---|
| 5AA / 6AA | 55–65% | Excellent — highest implantation potential |
| 5AB / 5BA | 45–55% | Very good — strong candidates for transfer |
| 4AA / 4AB | 40–50% | Good — slightly earlier stage but quality cells |
| 4BB / 5BB | 35–45% | Average — reasonable transfer candidates |
| 3BB / 4BC | 25–35% | Below average — lower but not negligible chance |
| 3BC / 3CC | 15–25% | Poor morphology — lowest success but pregnancies do occur |
A large 2022 retrospective study of over 5,000 single frozen embryo transfers found that while AA embryos had the highest implantation rates, BB and even BC embryos still resulted in healthy pregnancies 25–35% of the time. Morphology is a probability estimate, not a guarantee in either direction.
Common Grade Misconceptions
"My embryo is only a 3-day embryo, not a blastocyst — is it bad?"
No. Some embryos develop more slowly but are perfectly healthy. Day-3 (cleavage stage) embryos are graded differently — by cell number and fragmentation. A good day-3 embryo has 6–10 cells with less than 10% fragmentation. Some clinics still transfer at day 3 in specific situations.
"My embryo is a day-6 blast — is that worse than day 5?"
Slightly. Day-5 blastocysts have marginally higher implantation rates than day-6, but the difference is small (roughly 5–10%). Day-6 blasts routinely result in healthy pregnancies. Day-7 blastocysts are more debated — some clinics freeze them, others don't.
"I only have C-grade embryos — should I give up?"
Absolutely not. "C" quality embryos (particularly BC or CB) have lower per-transfer success rates, but they still result in pregnancies. If they're your only option, they're worth transferring. The grading system captures morphological appearance at one point in time — it doesn't measure the underlying genetics (which is what PGT-A does).
"My embryo was PGT-A normal — does the grade still matter?"
Less so. Once an embryo passes PGT-A, chromosomal normality is confirmed — the strongest predictor of implantation. Among euploid embryos, morphological grade has a smaller (but not zero) predictive impact. An euploid 4BB is typically preferred over an euploid 3BC, but both have strong chances.
Day 3 (Cleavage Stage) Grading
If your clinic evaluates or transfers at day 3, grading focuses on:
- Cell number: Ideal is 6–10 cells at day 3. Under 6 suggests slow development; over 10 can indicate irregular division.
- Fragmentation: Graded as percentage of embryo volume. <10% = Grade 1 (excellent). 10–25% = Grade 2 (good). >25% = Grade 3 (fair). >50% = Grade 4 (poor).
- Symmetry: Evenly sized cells (blastomeres) are preferred.
What to Ask Your Embryologist
When your clinic calls with your embryo report, these questions will help you understand your situation:
- How many embryos reached blastocyst stage? (This is your "blastocyst conversion rate" — 40–60% is typical)
- What are the grades of each embryo?
- Which embryo would you recommend transferring first, and why?
- Are any embryos borderline for freezing, and what's your clinic's policy?
- Do you use AI-assisted grading alongside traditional morphological assessment?
For how AI is changing embryo selection, see AI Embryo Selection. For supplements that support embryo development, visit CoQ10 for Fertility on LifeFertile.