What your embryo grade means (and what it doesn't)

Embryo grading is the lab's way of describing how an embryo looks so the team can decide which to transfer first and which to freeze. It is useful, it is honest, and it is also widely misunderstood. Here is what the codes mean and how much weight to put on them.

Day 3 (cleavage stage): cell number and appearance

On day 3 an embryo is a small cluster of cells. The embryologist records:

• Cell number — ideally 6–10 cells on day 3, with 8 the textbook figure. Fewer suggests slower development; many more can suggest uneven division.
• Fragmentation — small pieces of cell that have broken off. A little (under 10%) is common; a lot is a negative sign.
• Symmetry — whether the cells are roughly equal in size.

Many clinics summarise this as Grade 1 to 4 (or A to D), where 1/A is even cells and little fragmentation and 4/D is heavily fragmented or uneven. So "8-cell Grade 1" is as good as day 3 gets.

Day 5–6 (blastocyst): the three-part code

Most clinics grade blastocysts with the Gardner system, which gives one number and two letters — for example 4AA or 3BB.

The number (1–6): how far the blastocyst has expanded

Number	What it means
1–2	Early blastocyst — cavity just forming
3	Full blastocyst — cavity fills the embryo
4	Expanded — larger, shell thinning
5	Hatching — breaking out of the shell
6	Hatched — fully out of the shell

Higher generally means further along, but 3, 4 and 5 are all normal on day 5. A "6" is not better than a "4"; it is just later.

The first letter (A–C): the inner cell mass — the clump of cells that becomes the baby. A = many tightly packed cells; B = several, loosely grouped; C = few cells.

The second letter (A–C): the trophectoderm — the outer layer that becomes the placenta. A = many cells forming a neat cohesive layer; B = fewer, looser; C = very few, large cells.

So 4AA is an expanded blastocyst with an excellent inner cell mass and an excellent outer layer. 3BB is a full blastocyst with good-not-perfect cells in both. 5BC is hatching, with a good inner cell mass but a weaker outer layer.

How much grade matters

• Grade does predict chances on average: top-grade blastocysts implant more often than lower-grade ones. That's why the lab transfers the best-looking one first.
• Grade doesn't tell you whether an embryo is genetically normal. Beautiful embryos can carry chromosomal errors; plainer ones can be perfectly healthy. Only genetic testing (PGT-A) looks at that, and it has its own limits. PGT-A explained →
• Grade is a judgement, not a measurement. Two experienced embryologists agree most of the time, not all of the time. Labs work hard to keep their grading consistent, but don't compare a "4AA" from one clinic with a "4AA" from another as if they were the same number.
• Some C grades are still transferred, and pregnancies happen. A "C" trophectoderm is a real negative; a "C" inner cell mass more so. Your embryologist will tell you what they'd recommend.

Why labs care about consistency

Because grading drives decisions, good labs check that their embryologists grade the same way — periodically scoring the same set of images and comparing. It's an unglamorous piece of quality work that directly affects which embryo you receive first.