Embryo grading: what the letters and numbers actually mean

At some point after retrieval you will be sent something like “4AB” or “3BB” or “day 5, 5AA”, and the natural assumption is that you have been handed a score, with one end good and the other bad.

It is more specific and less final than that. Embryo grading is a description of what an embryologist sees through a microscope at a particular moment. It is a useful description, and it is not the whole picture.

What is being described

By day five or six after fertilisation, an embryo that is developing well has reached the blastocyst stage: a fluid-filled structure with two distinct cell groups inside. Most grading systems describe three things about it.

A number, usually 1 to 6: how expanded it is. The blastocyst grows and fills with fluid, then eventually breaks out of its outer shell. The number says where in that progression the embryo was when it was looked at. A higher number means further along — though “further along” is only meaningful relative to the day, since an embryo graded on day five and one graded on day six are at different points by definition.

A first letter: the inner cell mass. This is the group of cells that would become the baby. Graded roughly from A to C by how many cells there are and how tightly organised they look.

A second letter: the trophectoderm. The outer layer, which would become the placenta. Graded the same way.

So “4AB” is a reasonably expanded blastocyst, with an inner cell mass that looked good and an outer layer that looked slightly less uniform.

What the grades are good for

Grading exists to answer one practical question: given several embryos, which one should be transferred first? It is an ordering tool, and for that it works. Embryos that look better under the microscope do, on average, implant more often, which is why clinics use morphology to prioritise.

That is the whole of its job. It is a way of deciding sequence.

What grading does not tell you

It is not a genetic assessment. Appearance under a microscope and chromosomal makeup are different properties, and one does not reliably predict the other. This is precisely why preimplantation genetic testing exists as a separate option: it answers a question morphology cannot reach. If genetic testing is part of your plan, the grade and the test result are two different pieces of information about the same embryo, and your clinic will read them together.

It is not a prediction about your embryo. The relationship between grade and implantation is a pattern across large numbers of embryos, not a property of yours. Lower-graded embryos implant and become healthy babies; higher-graded ones sometimes do not. The average tells you how to order a queue, not what will happen in one case.

It is not fully standardised. Grading systems vary between laboratories, and within a system there is a degree of judgement. A “B” in one clinic is not guaranteed to be a “B” in another. This matters if you are comparing reports from different places, or moving embryos between clinics — a situation worth understanding before it arises, as we covered in moving frozen embryos and eggs across borders.

It is a snapshot. The embryo was graded at a moment. Laboratories that use time-lapse monitoring observe development continuously rather than at checkpoints, which is additional information rather than a different grade.

What to ask instead

Rather than asking whether a grade is good, ask questions the answers to which will actually shape what happens:

• Which grading system do you use, and what is the range? Without this, a letter means nothing on its own.
• How many embryos do I have, and how are you ordering them for transfer?
• What are you recommending we transfer first, and why that one?
• Are we testing the embryos, and how will that change the order?
• What does the rest of my picture suggest, beyond the grades?

That last question is the one that matters most. Embryo grade is one input. Your age, your history, the endometrium, the specific findings from this cycle — all of it goes into the recommendation, and the doctor making it is looking at considerably more than the letters.

If the grades disappoint you

They often do, because people read them as marks out of ten and almost nothing gets full marks. A handful of mid-graded blastocysts is a normal and workable outcome, not a poor one.

And if a transfer does not succeed, the grade is rarely the whole explanation. The review that follows looks at the cycle as a whole, which is the conversation that should shape the next attempt.

This article is general information, not medical advice. Your embryologist and your doctor can tell you what your specific report means in the context of your case.

For how the laboratory stage fits into the rest of treatment, read the patient journey.