Treatment · 4 min read
If you're going through IVF, you'll probably hear your embryos described with a mix of numbers and letters — something like '4AA' or '3BB'. Grading is simply a way for embryologists to describe how an embryo looks. It's a useful guide, not a guarantee.
Most UK clinics use grading systems based on consensus guidance from the European Society of Human Reproduction and Embryology (ESHRE). Day 3 (cleavage stage) embryos and Day 5 (blastocyst) embryos are graded slightly differently.
On day 3, embryologists describe how many cells the embryo has (ideally 6–8), how even those cells look, and how much fragmentation (small bits of cellular debris) is present. An "8-cell, grade 1" embryo means 8 even cells with minimal fragmentation.
On day 5 or 6, blastocysts are graded using a system developed by Gardner and Schoolcraft. You'll see something like "4AA":
The number (1–6) describes how expanded the blastocyst is — bigger numbers mean it has expanded more and may even be starting to "hatch" out of its shell.
The first letter (A–C) describes the inner cell mass, which becomes the baby. The second letter (A–C) describes the trophectoderm, which becomes the placenta. A is the highest quality, C is the lowest.
Grading is based on appearance only. It doesn't directly measure chromosomes, and it can't promise a pregnancy. Healthy babies are regularly born from embryos graded BB or BC, and not every "AA" embryo will implant. Research consistently shows that grading is helpful but not deterministic — it's one tool among several.
Embryologists use grading mainly to decide which embryo to transfer first, and which embryos are suitable to freeze for later use. The wider context — your age, history, embryo development pattern — matters just as much as the score itself. If your grades feel confusing, please ask your clinical team to walk you through them.
