What the Numbers and Letters in a Blastocyst Grade Actually Describe

If you have been given an embryo report, you may have seen a short code such as 4AA, 3BB or 5AB. It looks like a score, but it is closer to a description. A blastocyst grade records what an embryologist observed about an embryo’s shape and structure at a particular moment. It is a morphological description, not a measurement of the embryo’s chromosomes and not a prediction of whether a transfer will lead to a pregnancy.

The notation is compact because it packs three observations into a few characters. Once you know what each position refers to, the code becomes readable rather than mysterious.

The three parts of the notation

Most blastocyst grading systems used in IVF laboratories follow a similar logic. A leading number describes the embryo’s stage of development and expansion. Two letters follow: the first describes the inner cell mass, and the second describes the trophectoderm.

Position in the code	What it describes	What the reader is being told
Leading number	Expansion and hatching stage	How large the blastocoel cavity is and whether the embryo is beginning to hatch from its outer shell
First letter	Inner cell mass	How tightly packed and clearly defined the cluster of cells that will form the fetus appears
Second letter	Trophectoderm	How organised and continuous the outer cell layer that will form the placenta appears

So in a code like 4AB, the 4 refers to expansion, the A to the inner cell mass and the B to the trophectoderm. The letters are usually read from best to poorest appearance, commonly A, B and C, though the exact wording of each category is set by the laboratory.

What the leading number describes

The number reflects how far the blastocyst has developed. Early blastocysts have a small cavity; later ones have a large, expanded cavity; and the highest numbers describe an embryo that is hatching or has hatched from the zona pellucida, the protective outer layer.

Many laboratories use a scale running from 1 to 6, where 1 indicates an early blastocyst and 6 indicates a hatched blastocyst. The important point is that this number is about developmental stage, not quality in the abstract. A day-5 embryo and a day-6 embryo may sit at different points on the scale simply because they were observed at different times.

What the first letter describes

The inner cell mass is the group of cells that will develop into the fetus. The first letter records how that cluster looks: whether the cells are numerous, tightly packed and clearly visible, or fewer, looser or harder to make out. An A usually means a well-defined, compact inner cell mass; lower letters describe a less organised appearance.

What the second letter describes

The trophectoderm is the outer layer of cells that will contribute to the placenta. The second letter records how that layer looks: whether it forms a cohesive, orderly sheet of many cells, or whether it is sparse, irregular or patchy. Again, A generally describes the most organised appearance and lower letters describe progressively less organised ones.

Reading the code as a whole

The three parts are not added up into a single score. A 4AA and a 3AA share the same letter grades but describe embryos at different expansion stages. A 4AB and a 4BA both have one A and one B, but the letters refer to different structures, so they are not interchangeable.

When you read your own report, it helps to ask three separate questions: what stage was the embryo at, how did the inner cell mass look, and how did the trophectoderm look? Keeping the parts separate avoids the trap of treating the code as a single mark out of ten.

Why the same embryo can receive different grades

Grading is a judgement made by a person looking through a microscope at a specific time. Laboratories differ in how they define each category, when they observe embryos, and how they word their reports. Two laboratories looking at comparable embryos may therefore use different letters, and the same embryo observed on different days may move up the expansion scale as it grows.

This is one reason a grade should be read alongside the laboratory’s own key rather than compared directly with a code from a different clinic. If you are comparing explanations between laboratories, it is reasonable to ask each one how it defines its categories. A published guide on comparing embryo grading explanations between IVF laboratories walks through that comparison.

What a grade does and does not predict

A grade describes appearance. It does not read the embryo’s chromosomes, and it cannot confirm that an embryo is genetically normal or that a transfer will succeed. Morphology and genetic testing can tell different stories about the same embryo, which is why a good-looking embryo can still have an abnormal chromosome result and a lower-graded embryo can test normally.

If you are considering preimplantation genetic testing, the relevant question is separate from grading. PGT-A assesses chromosome number in cells sampled from an embryo, and the test has limitations: it can produce incorrect findings, and a result does not guarantee a successful pregnancy or a healthy child. Whether to use it is a decision to discuss for your individual situation with your treating clinician and a genetic counsellor. A guide to reading your PGT-A results after IVF explains how those reports are usually laid out, and a companion piece on embryo appearance versus genetic test results explores why the two kinds of information can diverge.

It is also worth remembering that a biopsy itself carries a risk: the HFEA notes that embryo biopsy can cause damage and prevent further development. That risk belongs in the conversation about whether testing is right for you, not in the grade itself.

Questions worth bringing to your clinician

You are entitled to ask your treating team to walk through your own report. Useful questions include:

• Which grading system does this laboratory use, and what does each number and letter mean in its key?
• On which day was this embryo observed, and how does that affect the expansion number?
• How does this grade relate to the decision you are recommending for my transfer?
• If PGT was performed, how should I read the grade and the genetic result together?

These are clinical questions, and they are best answered by the clinicians who know your history and the laboratory’s conventions. A coordination service can help you organise records and prepare questions, but it does not interpret results or recommend treatment.

Keeping your records straight

Embryo reports often arrive as a short message or a PDF with several codes. Before a consultation, it helps to note the date of each report, the day of embryo development, and the exact code as written, rather than a remembered summary. If you are preparing for a remote consultation from abroad, a dated record checklist and a list of outstanding documents make the conversation more efficient. A broader set of questions to ask about embryo quality can help you decide what matters most to you.

None of this changes the core point: the numbers and letters are a description of appearance, written in shorthand. They are useful information, but they are not a verdict.

Sources: Embryo biopsy can cause damage and prevent further development. — HFEA PGT-A frequently asked questions (checked 2026-10-15).

Frequently asked questions

Does a higher blastocyst grade mean the embryo is genetically normal?

No. A grade describes how the embryo looks under a microscope. It does not assess chromosomes. Genetic status is addressed by a separate test such as PGT-A, which has its own limitations and should be discussed with your clinician and a genetic counsellor.

Why did my embryo's grade change between reports?

The leading number reflects developmental stage, so an embryo can move along the expansion scale as it grows between observations. Letter grades are also judgements made by an embryologist at a particular time, and laboratories define their categories differently.