Embryo Grading Explained: What 4AA, 5AB, and Those Letters Actually Mean

Your embryologist calls.

“You have a 4AA, a 4BB, and a 5AB.”

You nod like a person who absolutely knows what any of that means.

Then you immediately Google it.

Quick answer: Blastocyst grades describe how expanded the embryo is and how two visible cell groups look under the microscope. They are useful for ranking embryos, but they do not guarantee implantation, live birth, or chromosome status.

First: embryo grading is not a school report card

The grade is a morphology description.

In plain English:

What does this embryo look like right now?

It is not:

• a certainty score
• a baby-quality rating
• a guarantee
• a judgment about whether the embryo is “good enough”

What does the number mean?

In a common blastocyst grading system, the number describes how expanded the blastocyst is.

As a blastocyst develops, it expands and may eventually begin hatching from its outer shell.

Labs can vary in exactly how they use these categories, so treat the number as a lab description, not a universal mathematical score.

What does the first letter mean?

The first letter describes the inner cell mass.

That is the cluster of cells that contributes to the fetus.

What does the second letter mean?

The second letter describes the trophectoderm.

Those cells contribute to placental tissues.

So in a grade like:

4AB

• 4 = expansion stage
• A = inner cell mass appearance
• B = trophectoderm appearance

Is an AA embryo guaranteed to work?

No.

This is important enough to say twice.

No.

A gorgeous-looking embryo can fail to implant.

A lower-graded embryo can lead to a healthy live birth.

Morphology gives the clinic useful information, but it cannot show everything that determines embryo viability.

Does grading tell you whether the embryo is chromosomally normal?

Also no.

Appearance and chromosome status are not the same thing.

A highly graded embryo can be aneuploid. A less impressive-looking embryo can be euploid.

PGT-A attempts to provide information about chromosome copy number from biopsied cells, but PGT-A has its own limitations too.

We get into that here: PGT-A in IVF: what it can and cannot tell you.

Why do grades seem different between clinics?

Because embryo grading is not perfectly standardized across every IVF lab.

Two clinics may describe embryos somewhat differently.

That is why comparing your “4BB” with a stranger’s “4BB” online is not quite as precise as it sounds.

The lab itself matters.

“Which embryo would you transfer first?”

Clinics may combine:

• morphology
• development day
• expansion
• PGT results, if available
• prior outcomes
• their own laboratory data

If you have several embryos, ask your clinic how they prioritize them.

That is much more useful than trying to reverse-engineer a universal ranking chart from the internet.

💡 What this means

Embryo grading helps answer:

Which embryo looks more developmentally promising based on what we can see?

It does not answer:

Will this embryo definitely become a baby?

That distinction will save you a lot of unnecessary panic.

FAQ

Is 4AA the best possible embryo?
It is generally considered a strong morphology grade in common grading systems, but it still does not guarantee success.

Can a lower-graded embryo work?
Yes.

Does PGT replace grading?
No. They assess different things.

Sources

• ASRM, Embryology Laboratory Guidance: https://www.asrm.org/practice-guidance/practice-committee-documents/comprehensive-guidance-for-human-embryology-andrology-and-endocrinology-laboratories-management-and-operations-a-committee-opinion-2022/
• ASRM, PGT-A Committee Opinion: https://www.asrm.org/practice-guidance/practice-committee-documents/the-use-of-preimplantation-genetic-testing-for-aneuploidy-a-committee-opinion-2024/