Embryo Grading Explained: AA, AB, BB & Blastocyst Grades

Few things in IVF cause as much confusion as an embryo grade. You are handed a code like 4AA, 3BB or 5BC, often with no explanation, and left to work out whether it is good news. Couples then search for that code, find a chart, and conclude that a BB embryo is a poor one. Usually it is not.

This article explains what each part of a blastocyst grade means, how the grades compare, and, most importantly, what a grade can and cannot tell you about your chance of a baby.

For where grading fits into the days after egg retrieval, our article on what happens after egg retrieval covers the full laboratory timeline.

What Embryo Grading Actually Measures

Embryo grading is a visual assessment. An embryologist looks at each embryo under a microscope and scores how it appears: how far it has developed, how many cells it has, and how those cells are organised.

That is all it measures. Grading does not test chromosomes, genes or any other biological property of the embryo. It is a structured way of describing appearance, and appearance correlates with outcome on average, but imperfectly in any individual case.

Keep that distinction in mind through everything that follows.

Day 3 Grading

Embryos transferred or assessed on day 3 are at the cleavage stage, typically six to eight cells. Grading at this stage looks at three things:

• Cell number. Around eight cells on day 3 is considered on track. Noticeably fewer suggests slow development.
• Symmetry. Whether the cells are evenly sized.
• Fragmentation. Small pieces of cellular debris between the cells, usually expressed as a percentage. Less is better.

Grading scales at this stage vary considerably between laboratories, using numbers, letters or descriptive terms. A day 3 report reading something like "8-cell, grade 1, under 10 percent fragmentation" describes a good-quality cleavage-stage embryo.

Day 3 grading is less predictive than blastocyst grading, because many embryos that look good on day 3 stop developing on day 4 or 5 when the embryo's own genome takes over. That is one reason most centres now culture to blastocyst.

Blastocyst Grading: The Gardner System

Most laboratories grade blastocysts using the Gardner system, or a close variant of it. A blastocyst grade has three parts, written as a number followed by two letters, for example 4AB.

1. The number describes how expanded the blastocyst is.
2. The first letter grades the inner cell mass, which becomes the baby.
3. The second letter grades the trophectoderm, which becomes the placenta.

The Number: Expansion Stage

A higher number means more developmental progress by the time of assessment. Stages 3 to 6 are generally considered suitable for transfer or freezing. The letters are usually only assigned from stage 3 onward, since earlier stages are not developed enough to assess.

The First Letter: Inner Cell Mass

The Second Letter: Trophectoderm

How the Grades Compare

Decoding a few common examples:

• 4AA: expanded blastocyst, top-grade inner cell mass and trophectoderm
• 3BB: full blastocyst, good inner cell mass and trophectoderm
• 5AB: hatching blastocyst, top inner cell mass, good trophectoderm
• 4BC: expanded blastocyst, good inner cell mass, fair trophectoderm

Which Matters More: the First Letter or the Second?

This is debated, and worth knowing because it is often presented as settled.

Some studies have found that trophectoderm grade, the second letter, is the stronger predictor of outcome in single blastocyst transfers. More recent work in frozen single transfers has found the inner cell mass, the first letter, carries more weight. Both components matter, and neither has been shown to dominate reliably across all settings.

The practical takeaway is that a strong grade in one component with a moderate grade in the other is still a good embryo, and embryologists weigh both together rather than ranking embryos on a single letter.

Day 5, Day 6 and Day 7 Blastocysts

Blastocysts reach the stage at different speeds. Day 5 blastocysts have, on average, slightly higher implantation rates than day 6. Day 6 blastocysts are entirely usable and result in many healthy pregnancies. Some laboratories also freeze day 7 blastocysts, which have lower average rates but are not without potential.

The day the embryo became a blastocyst is usually recorded alongside its grade and is part of how the laboratory prioritises which embryo to transfer first.

What a Grade Cannot Tell You

This is the most important section of the article.

A grade cannot see chromosomes. A 4AA blastocyst can be chromosomally abnormal, and this is the single most common reason a top-graded embryo fails to implant. Equally, a 4BC blastocyst can be chromosomally normal and go on to become a healthy baby. Only PGT-A can assess chromosome number, and our article comparing PGT-A, PGT-M and PGT-SR explains when that is appropriate.

A grade reflects one moment. Embryos are graded at a specific time, and their appearance continues to change. A blastocyst graded 3BB in the morning may look like a 4AB a few hours later.

Grading is subjective. Two experienced embryologists can grade the same embryo differently, and grading conventions vary between laboratories. A BB at one centre is not necessarily identical to a BB at another.

Age matters more than grade. The chance that any embryo is chromosomally normal depends heavily on the age of the woman when the eggs were retrieved. A BB embryo from a 31-year-old may well have better prospects than an AA embryo from a 42-year-old. Our article on IVF success rates by age explains why.

Does a Lower Grade Mean You Should Not Transfer?

Generally, no.

Higher-grade blastocysts do have higher implantation rates on average, and when several embryos are available, laboratories usually transfer the best-graded one first. But lower-grade blastocysts still result in healthy pregnancies, and the babies born from them are not less healthy than those born from higher-grade embryos.

If your only embryo is a 3BC or 4CB, it is still worth transferring. If your clinic has decided an embryo is suitable to freeze, it has judged that embryo to have genuine potential.

Time-Lapse Imaging and AI Grading

Some laboratories use time-lapse incubators, which photograph embryos continuously, and increasingly AI tools that score embryos from those images. These provide more information about how an embryo developed over time, rather than a single snapshot.

They are often marketed as add-ons. The evidence that they meaningfully improve live birth rates, compared with standard grading by an experienced embryologist, remains limited. They may help laboratories organise and prioritise embryos, but they do not change what an embryo is. As with any add-on, it is fair to ask what evidence supports it in your situation and what it costs.

Questions to Ask About Your Embryo Report

• What day did each embryo reach blastocyst?
• What grading system does your laboratory use?
• Which embryo will be transferred first, and why?
• Are lower-graded embryos being frozen, and if not, why not?
• Given my age, is genetic testing worth considering?

If an embryo has failed to implant despite a high grade, that is not a mystery, and it is not a reason to lose confidence in the remaining embryos. Our articles on what causes failed IVF and tests to consider after repeated failure cover what to look at next. If your embryos are frozen, our article on frozen embryo transfer explains the next stage.

You can read more on the IVF and PGT treatment pages, and see the full range of treatment options here.

You can book a free consultation with Dr. Rashmi Agrawal at the Sector 27, Gurugram clinic to go through your embryology report in detail, read more about her background and approach, or check the FAQ page.

FAQs

What does 4AA mean in embryo grading?

It describes an expanded blastocyst (stage 4) with a top-grade inner cell mass (A) and a top-grade trophectoderm (A). It is the highest common appearance grade, but it does not confirm the embryo is chromosomally normal.

Is a BB embryo good?

Yes. BB is a good-quality grade and one of the most common grades among embryos that result in healthy babies.

Can a low-grade embryo lead to a successful pregnancy?

Yes. Lower-grade blastocysts have lower implantation rates on average, but they still result in healthy pregnancies, and babies born from them are not less healthy.

Why did my AA embryo fail to implant?

The most common reason is a chromosomal abnormality, which grading cannot detect. A top grade describes appearance, not genetic normality.

Is a day 6 blastocyst worse than a day 5?

Day 6 blastocysts have slightly lower implantation rates on average, but they are entirely usable and result in many healthy pregnancies.

What is the difference between the inner cell mass and trophectoderm?

The inner cell mass becomes the baby. The trophectoderm becomes the placenta. Both are graded A, B or C based on cell number and organisation.

Does embryo grading vary between clinics?

Yes. Grading is a visual, subjective assessment, and conventions vary between laboratories. The same embryo can be graded slightly differently by different embryologists.

Does a high grade mean I don't need PGT-A?

No. Grading and genetic testing answer different questions. Whether PGT-A is worthwhile depends mainly on your age and history, not on embryo grades.

References

1. ESHRE and ALPHA Scientists in Reproductive Medicine. The Istanbul consensus update: a revised consensus on oocyte and embryo static and dynamic morphological assessment. Human Reproduction, 2025. https://academic.oup.com/humrep/article/40/6/989/8120385
2. Trophectoderm grade, but not inner cell mass grade, is predictive of outcomes in single blastocyst transfers. Fertility and Sterility. https://www.fertstert.org/article/S0015-0282(12)01094-1/fulltext
3. Revisiting embryo selection: inner cell mass outweighs the trophectoderm in predicting live birth in single frozen blastocyst transfers. https://pubmed.ncbi.nlm.nih.gov/41955832/