Embryo Grading: What the Grades Do and Don't Tell You

A few days after your egg retrieval, someone rings and reads you a short list. Four embryos. One 4AA, one 4AB, two 3BB. Then the call ends, and you are left holding a set of codes nobody has explained, trying to work out from the tone of voice whether that was good news.

Embryo grading is one of the more anxious parts of an IVF cycle, largely because it looks like a school report. Letters and numbers, higher apparently better, and an obvious temptation to read it as a verdict on whether this will work.

It is not that. Here is what the grades actually describe, and just as importantly what they do not.

What embryo grading actually measures

An embryologist looks at each embryo under a microscope and records what they can see. That is the whole of its appearance and rate of development, observed at a particular moment.

It is a visual assessment, not a test. Nothing is sampled, nothing is measured chemically, and no genetic information is obtained. The embryologist is describing how an embryo looks compared with how embryos at that stage usually look.

That sounds crude, and in a sense it is. But appearance does correlate with how likely an embryo is to continue developing, which is why grading is useful for deciding which embryo to transfer first when there is more than one. That is its purpose: ranking, not predicting.

Grading helps choose between embryos. It does not tell you whether any particular embryo will become a pregnancy.

Day 3 grading: the cleavage stage

By the third day after fertilisation, an embryo has divided into a small number of cells typically six to eight. At this stage the embryologist looks at three things.

Cell number. Whether the embryo has divided at roughly the expected rate. An embryo with eight cells on day three is developing on schedule; one with four is behind, and one with sixteen is ahead. Both extremes are noted.

Cell symmetry. Whether the cells are of similar size to one another. Evenly sized cells are generally a better sign than markedly uneven ones.

Fragmentation. Small pieces of cellular material that break away during division. A little is normal and common. A great deal makes the embryo harder to assess and is associated with lower rates of continued development.

Day 3 grades are usually written as a number followed by a grade or percentage something like "8-cell, Grade 1" or "8-cell, 10% fragmentation". The exact notation varies between laboratories, which is a theme we will come back to.

Day 5 grading: the blastocyst

By day five, an embryo that is still developing has become a blastocyst, a structure with a fluid-filled cavity, an inner group of cells, and an outer layer. This is a more informative stage to assess, because the embryo has done more and the structures being graded go on to become different things.

Blastocyst grading has three parts, usually written together as a number and two letters.

Expansion (the number, 1 to 6). How far the blastocyst has expanded, and whether it has begun to hatch out of its outer shell. Higher numbers mean further along.

Inner cell mass (the first letter, A to C). The cluster of cells that would go on to form the fetus. Graded on how many cells there are and how tightly packed they appear.

Trophectoderm (the second letter, A to C). The outer layer, which would go on to form the placenta. Graded on how many cells form it and how even that layer is.

So, the letters are not a mark out of ten. They describe two different parts of the embryo, and either can be strong while the other is less so.

So, what does 4AA mean?

An expanded blastocyst, with a well-formed inner cell mass and a well-formed outer layer. On appearance, about as good as it gets.

And a 3BB is a slightly less expanded blastocyst with both parts graded as adequate rather than excellent. Which sounds worse, and is a perfectly ordinary embryo that clinics transfer routinely and from which babies are routinely born.

This is where the school-report instinct causes unnecessary distress. B is not a failure. C is not a failure. These are descriptions of appearance within a normal range, not marks against a pass threshold.

Why grades are not predictions

A high-grade embryo is more likely to implant than a low-grade one. That is a statement about averages across many cycles, and it is true.

It says very little about your four embryos.

Good-looking embryos fail to implant. Ordinary-looking embryos produce healthy pregnancies. Clinics see both every week, and any embryologist will tell you the same thing. Grading shifts the odds a little; it does not determine the outcome.

The practical consequence is worth stating plainly: a report full of B grades is not a reason to despair, and a 4AA is not a guarantee. Neither deserves the weight people place on them at three in the morning.

Why clinics grade differently

There is no single universal grading system. Several are in common use, laboratories adapt them, and the notation differs.

This matters if you are comparing clinics, or moving between them. A grade from one laboratory is not directly comparable with a grade from another, and a set of embryos Regraded elsewhere may come back described differently without anything having changed.

There is also an unavoidable element of judgement. Two experienced embryologists looking at the same embryo may record slightly different grades. Good laboratories manage this with internal standards and regular cross-checking, but it is a visual assessment made by a person, and it carries the variability that implies.

What grading cannot see

This is the most important section, and the one most often left out.

Grading is appearance. It cannot see the chromosomes. An embryo that looks excellent may carry a chromosomal abnormality that will stop it developing; an ordinary-looking embryo may be entirely normal. Appearance and genetic status are related, but loosely.

Pre-implantation genetic testing is the only way to assess chromosomes directly, and it is a separate procedure with its own considerations not something grading substitutes for.

Nor does grading say anything about what happened before fertilisation. The quality of the egg and the sperm that made the embryo is not visible in the result. Where sperm DNA fragmentation has affected the embryo, that damage does not necessarily show in how the embryo looks on day five it more often shows later, in development that stops after an apparently good start. This is one of the reasons a thorough assessment of the male side before the cycle is worth the effort it takes.

What actually matters on that phone call

Three questions are more useful than the grades themselves.

How many embryos reached the blastocyst stage? The number that got that far tells you more than the letters attached to them.

Which one is being transferred first, and why? The reasoning matters more than the grade.

And what happens to the others? Whether they are suitable for freezing, and what that means for a future cycle without repeating stimulation.

Beyond that, the grades will not change, and reading them repeatedly will not make them more informative. It is a difficult few days, and there is genuinely nothing you can do to influence what is happening in the incubator.

Frequently asked questions

Is a 3BB embryo good enough to transfer?

Yes. 3BB describes a blastocyst with adequate inner cell mass and outer layer. Clinics transfer embryos of this grade routinely, and healthy pregnancies result from them regularly.

What is the best embryo grade?

On appearance, a fully expanded blastocyst with both cell groups graded A commonly written 4AA or 5AA. But the best-looking embryo is not guaranteed to implant, and a lower-graded one often does.

Does a low grade mean the embryo is abnormal?

No. Grading assesses appearance, not chromosomes. A lower-graded embryo may be chromosomally normal and a high-graded one may not be. Only genetic testing assesses chromosomes directly.

Why did my embryos stop developing after day three?

Some embryos are arrested between day three and day five. It is common and usually reflects an underlying problem with that embryo rather than anything done during the cycle. Growing to day five is partly a way of identifying this before transfer.

Can embryo grades change between days?

Yes. An embryo graded on day three may look quite different by day five, in either direction. Grades describe a moment, not a trajectory.

Do different clinics use the same grading system?

No. Several systems are in use and laboratories adapt them. Grades from different clinics are not directly comparable.

Should I ask for the grades of all my embryos?

You can, and most clinics will tell you. Whether it helps is another matter for many people. The numbers add anxiety without adding anything they can act on.

The short version

Grading describes how an embryo looks, at one moment, under a microscope. It helps the laboratory decide which embryo to transfer first. It is not a score, not a prediction, and not comparable between clinics.

A B is not a failure. A 4AA is not a promise. And the embryo that becomes your child may well not be the one with the best letters.

Conclusion

Grading describes how an embryo looks, at one moment, through a microscope. It helps the laboratory decide which to transfer first. That is all it does.

It is not a score, not a prediction, and not comparable between clinics. A B is not a failure. A 4AA is not a promise. And the embryo that becomes your child may well not be the one with the best letters.

One question grading cannot answer: it sees appearance, not chromosomes, and says nothing about the egg and sperm that made the embryo. Where development stops after a good start, the cause often sits further back which is why assessing the male side before the cycle matters more than couples expect.

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This article is for general information and is not a substitute for individual medical advice. Grading systems, culture practice and laboratory protocols vary between clinics; follow the guidance given to you by the team treating you.