You just received your embryo report from the IVF lab. It says something like: "5AA, 4BB, 3BC."
What do these numbers and letters mean? Is 5AA perfect? Is 3BC a failure? Should you feel hopeful — or worried?
If you're an intended parent pursuing surrogacy, these grades can feel like a verdict on your entire journey. But here's what you need to know upfront: embryo grading is a helpful tool, not a crystal ball. It tells you something meaningful about how an embryo looks and develops, but it doesn't determine destiny.
Key Takeaways
- Blastocyst grading uses the Gardner system: a number (1–6) for expansion stage, plus two letters (A–C) for inner cell mass and trophectoderm quality. A grade like "4AB" means expanded blastocyst, excellent ICM, good TE.
- Higher-grade embryos have better statistical odds, but lower-grade embryos can and do produce healthy babies. A 3BC is not a "failed" embryo.
- Both ICM and TE grades matter. Research is split on which is more predictive of live birth — some large studies favor TE, others favor ICM. The safest approach is to consider both grades together.
- Development day matters independently. Day 5 blastocysts generally outperform Day 6 and Day 7, but a high-grade Day 6 can outperform a low-grade Day 5.
- Grading evaluates appearance only. It cannot detect chromosomal abnormalities — that requires PGT-A testing. A euploid (chromosomally normal) 4BB may outperform a non-tested 5AA.
What Is a Blastocyst?
After fertilization, embryos are cultured in the lab for several days. By Day 5 to Day 7, a healthy embryo develops into a blastocyst — a structure of roughly 100–200 cells with two distinct parts:
- Inner Cell Mass (ICM): The cluster of cells that will become the fetus.
- Trophectoderm (TE): The outer ring of cells that will become the placenta.
Between them sits a fluid-filled cavity (blastocoel) that expands as the embryo grows, eventually helping it "hatch" from its protective shell to implant in the uterine lining.
Most modern IVF programs culture embryos to this stage before transfer or freezing, because blastocysts provide significantly more information about developmental potential than earlier-stage embryos. If you're a surrogate preparing for a Day 5 transfer, our 5-day embryo transfer timeline guide walks you through what to expect.
How Blastocyst Grading Works: The Gardner System
The global standard for evaluating blastocyst quality is the Gardner grading system, developed by embryologist Dr. David Gardner in 1999 and endorsed as a reference framework by the American Society for Reproductive Medicine (ASRM).
Each blastocyst receives a three-part score — for example, 4AB or 5AA.
The Number (1–6): Expansion Stage
This reflects how far along the blastocyst is in its development.
Grade | Stage | What It Means |
|---|---|---|
1 | Early blastocyst | Cavity fills less than half the embryo |
2 | Blastocyst | Cavity fills more than half |
3 | Full blastocyst | Cavity completely fills the embryo |
4 | Expanded | Shell (zona pellucida) begins to thin |
5 | Hatching | Embryo breaking through its shell |
6 | Hatched | Embryo fully escaped from shell |
Expansion scores of 3–6 are generally considered transferable. A higher number reflects more advanced development, but doesn't automatically mean a better embryo — because when an embryo reaches blastocyst stage (Day 5 vs. Day 6) matters too.
The First Letter (A, B, or C): Inner Cell Mass Quality
Grade | Description |
|---|---|
A | Tightly packed, many cells, well-defined |
B | Loosely grouped, moderate number of cells |
C | Very few cells, difficult to distinguish |
The Second Letter (A, B, or C): Trophectoderm Quality
Grade | Description |
|---|---|
A | Many cells forming a cohesive, even layer |
B | Fewer cells, some irregularity in size or spacing |
C | Very few cells, gaps or fragmentation |
Reading Your Grade
A grade of 4AB means: expanded blastocyst (4), excellent ICM (A), good TE (B).
A grade of 5AA — hatching blastocyst with top marks in both ICM and TE — is the highest grade commonly seen. A grade of 3CC — full blastocyst with poor ICM and TE — is the lowest that might still be considered for transfer.
Gardner Grades and the SART Classification
If your clinic report uses terms like "Good," "Fair," or "Poor" instead of (or alongside) the alphanumeric Gardner grade, they're using the SART classification — a simplified system developed by the Society for Assisted Reproductive Technology to standardize reporting across U.S. clinics.
SART doesn't replace Gardner grading. It groups Gardner grades into three tiers:
SART Category | Gardner Grades Included | What It Means |
|---|---|---|
Good | 4AA, 4AB, 4BA, 5AA, 5AB, 5BA, 3AA, 3AB, 3BA, and similar combinations where both ICM and TE are A or B with at least one A | Embryo with minimal imperfections |
Fair | 4BB, 5BB, 3BB, and similar combinations where both ICM and TE are B | Viable embryo, moderate quality |
Poor | Any combination with a C in either ICM or TE (e.g., 4BC, 3CB, 5CC) | Lower quality, but not necessarily unviable |
Some clinics further refine this with plus (+) or minus (−) modifiers — for example, "Good−" or "Fair+" — to distinguish embryos within the same tier. These modifiers are internal clinic adaptations rather than part of the official SART standard, but they can help your fertility team rank embryos more precisely when multiple embryos share the same SART category.
Reading a PGT-A Lab Report: Common Abbreviations
If you've received a PGT-A report, you may notice yet another format — many genetic testing labs use abbreviations for the expansion stage combined with SART-style quality descriptors. For example, a report might list an embryo as "FB F" or "XB G+" instead of "3BB" or "4AA."
Here's how these abbreviations map to the Gardner system:
Lab Abbreviation | Full Name | Gardner Number |
|---|---|---|
EB | Early Blastocyst | 1–2 |
FB | Full Blastocyst | 3 |
XB or EXB | Expanded Blastocyst | 4 |
HgB | Hatching Blastocyst | 5 |
HdB | Hatched Blastocyst | 6 |
The letter after the abbreviation is the SART-style quality rating — G (Good), F (Fair), or P (Poor) — sometimes with +/− modifiers:
Quality Code | Meaning | Approximate Gardner ICM/TE |
|---|---|---|
G or G+ | Good (top quality) | AA, AB, or BA |
G− | Good (lower end) | BA, AB borderline |
F+ | Fair (upper end) | BB with strong features |
F | Fair | BB |
F− | Fair (lower end) | BB with weaker features |
P | Poor | Any C grade (BC, CB, CC) |
So when a PGT-A report says "FB F", it means: Full Blastocyst (Gardner 3), Fair quality (approximately BB) — equivalent to roughly a 3BB in the Gardner system. An "XB F" would be an Expanded Blastocyst, Fair quality — roughly a 4BB.
The +/− modifiers give embryologists finer granularity within each tier. For example, an "FB F−" (as seen in some reports) indicates a Full Blastocyst that falls on the lower end of Fair — perhaps with slightly weaker ICM or TE organization than a standard "FB F."
Confused by your clinic's report? You don't have to decode it alone. Contact the Ivy Surrogacy team for guidance on understanding your embryo results and planning your next steps.
What the Research Says About Grades and Success Rates
Higher grades correlate with better outcomes
A study of 717 single blastocyst transfers (Heitmann et al., 2013) using the SART classification found live birth rates of 53% for Good, 37% for Fair, and 28% for Poor quality embryos. A separate analysis of over 223,000 transfers reported similar patterns.
Which matters more — ICM or TE?
Intuitively, the inner cell mass should matter most — after all, it becomes the baby. But the research is surprisingly split on this question, and the debate remains unresolved.
Several studies have found that trophectoderm quality is the stronger independent predictor of live birth. The logic: the TE is the part of the embryo that directly contacts the uterine lining during implantation and later forms the placenta, so a well-organized TE may be critical for the embryo to successfully "take hold." A 2024 study of 1,546 frozen transfers (Bartolacci et al., Life) found that TE was more predictive than either ICM or expansion status after adjusting for confounders.
However, other large studies have reached the opposite conclusion. A 2021 analysis of 10,482 frozen single blastocyst transfers (Ai et al., Frontiers in Endocrinology) found that ICM was the strongest predictor — live birth rates dropped from 54.6% (ICM grade A) to 28.5% (ICM grade C), with ICM remaining significant in multivariate analysis while TE lost significance after adjustment.
The bottom line for intended parents: The research is genuinely mixed, and neither component has been definitively proven superior. The safest approach — and the one most clinicians follow — is to consider both grades together. An embryo with strong marks in both ICM and TE (e.g., AA or AB) is preferable to one that is strong in only one dimension.
Lower grades still produce healthy babies
This is the most important takeaway: lower-graded embryos can and do result in healthy pregnancies. A 3BC has lower statistical odds than a 5AA, but it is absolutely not a "failed" embryo. Many families worldwide have welcomed healthy children from embryos classified as Fair or Poor.
Development Day Matters Too
The day an embryo reaches blastocyst stage is an independent predictor of success.
A 2025 meta-analysis combining 49 studies and over 106,000 frozen transfers (Hou et al.) found that Day 5 blastocysts consistently had higher pregnancy and live birth rates than Day 6, both with and without PGT-A. A large dataset of 13,830 euploid transfers reported live birth rates of 55% for Day 5, 44% for Day 6, and 21% for Day 7. While Day 7 embryos have the lowest odds, a euploid Day 7 blastocyst still offers a real chance at pregnancy and should not be discarded if no other options are available.
However, when only a low-grade Day 5 and a high-grade Day 6 are available, research shows the high-grade Day 6 may be the better choice (50.4% vs. 41.2% live birth rate). Morphological quality can sometimes override developmental speed.
Your fertility team weighs both factors — grade and day — together with PGT-A results when selecting which embryo to transfer. You can estimate your expected outcome using our Embryo Transfer Success Rate Calculator.
Grading vs. PGT-A: A Crucial Distinction
Embryo grading evaluates what an embryo looks like on the outside. PGT-A analyzes the actual chromosomes inside.
A beautiful 5AA embryo can be chromosomally abnormal and fail to implant. Conversely, a modest 4BB confirmed euploid by PGT-A may have excellent chances. Chromosomal status is a stronger predictor of live birth than morphological grade alone — but morphology still adds value even among euploid embryos, since high-grade euploid blastocysts still outperform low-grade euploid blastocysts.
The most complete picture comes from combining both. For a deeper dive into how PGT-A works and what the results mean, see our complete PGT-A guide for IVF and surrogacy.
What Grading Can't Tell You
- Grading is subjective. Two embryologists may assign slightly different grades to the same embryo. A "4AB" at one clinic could be a "4BB" at another.
- Grading is a snapshot. An embryo graded 3BB might look like 4AB six hours later. The grade captures one moment in a dynamic process.
- Grading can't detect chromosomal problems. Only PGT-A or similar genetic testing can reveal whether an embryo is euploid.
What This Means for Your Surrogacy Journey
Don't panic over grades. A mix of 5AA, 4BB, and 3BC is actually a strong position. Even the 3BC deserves consideration.
Trust the combined picture. The best decisions come from weighing morphology, development day, and PGT-A results together. No single data point tells the whole story.
Cumulative success is real. If a first transfer doesn't succeed, the odds improve with each subsequent attempt — especially with multiple euploid embryos available. If you've experienced an unexpected negative result, our guide on why good-quality embryos sometimes fail to implant explains the science behind it.
Frequently Asked Questions
1. What is the best blastocyst grade for embryo transfer?
Grades like 4AA, 5AA, or 5AB are associated with the highest success rates (roughly 50–65% live birth per transfer). But "best" also depends on development day and PGT-A status — a euploid 4BB on Day 5 may outperform a non-tested 5AA.
2. Can a low-grade embryo still result in a healthy pregnancy?
Yes. Studies show live birth rates of 25–35% for Fair-quality embryos, and pregnancies do occur from Poor-grade blastocysts. If the embryo is confirmed euploid, chances improve further.
3. Should Day 5 embryos always be transferred before Day 6?
Generally yes, but when only a low-grade Day 5 and a high-grade Day 6 are available, the high-grade Day 6 may be the better choice. Your fertility team will decide based on your specific embryo cohort.
4. Does embryo grading differ between clinics?
Yes. Most clinics use the Gardner system, but some add extra categories or use simplified scales. PGT-A labs often use abbreviations like FB, XB, or HgB combined with Good/Fair/Poor ratings. The SART system provides standardized reporting categories, but some variation still exists. Always ask your clinic to explain their specific criteria.
5. If 6 is the highest expansion number, why is 5AA — not 6AA — considered the top grade?
A grade of 6 means the embryo has fully hatched — it has completely escaped from its protective shell (zona pellucida). While this sounds like the most advanced stage, in practice most embryos are biopsied, frozen, or transferred at expansion stages 4–5. There's a practical reason for this: when PGT-A is performed, embryologists need to remove a few cells from the trophectoderm for genetic analysis. A hatching blastocyst at stages 4–5 provides the ideal window — the TE is accessible, the embryo is structurally stable, and the biopsy can be performed safely before the embryo is vitrified to await results. By the time an embryo reaches stage 6, it has fully escaped its shell, making it more fragile to handle and harder to assess. So 6AA embryos do exist and can be excellent — but they're uncommon in clinical reports because most labs complete grading, biopsy, and cryopreservation before the embryo reaches that point.
References
- Gardner DK, Lane M, Stevens J, et al. Blastocyst score affects implantation and pregnancy outcome: towards a single blastocyst transfer. Fertility and Sterility. 2000;73(6):1155-1158. PubMed
- Heitmann RJ, Hill MJ, Richter KS, et al. The simplified SART embryo scoring system is highly correlated to implantation and live birth in single blastocyst transfers. Journal of Assisted Reproduction and Genetics. 2013;30(4):563-567. PubMed
- Ai J, Jin L, Zheng Y, et al. The morphology of inner cell mass is the strongest predictor of live birth after a frozen-thawed single embryo transfer. Frontiers in Endocrinology. 2021;12:621221. (10,482 cycles.) Full Text
- Bartolacci A, de Girolamo S, Solano Narduche L, et al. Trophectoderm, inner cell mass, and expansion status for live birth prediction after frozen blastocyst transfer: the winner is trophectoderm. Life. 2024;14(11):1360. (1,546 cycles.) Full Text
- Hou W, Wang Z, Wen T, et al. Pregnancy and perinatal outcomes after day 5 versus day 6 blastocyst-stage embryo transfer: a systematic review and meta-analysis. Acta Obstetricia et Gynecologica Scandinavica. 2025. doi:10.1111/aogs.70090. (49 studies; 106,316 cycles.) PubMed
At Ivy Surrogacy, we understand how overwhelming IVF reports can feel — especially when you're navigating this journey across borders and time zones. Our team is here to help you understand your embryo results, work with your fertility clinic, and plan your next steps with clarity and confidence. Start your surrogacy journey today.
