How are embryos graded, and what do the letters mean?
7 min read · updated September 2026
Short answer:A day-5 blastocyst grade has three parts: a number from 1 to 6 for how expanded it is, then two letters (A, B or C) for the quality of the cells that become the baby and the cells that become the placenta. So 4AA is an expanded blastocyst with good cells in both groups. Grading describes appearance on one morning — it predicts odds, it does not decide them.
Embryo grading is a lab shorthand for how an embryo looks down a microscope at a particular moment. It is genuinely useful — it is how embryologists decide which embryo to transfer first — and it is also the single most over-interpreted number in IVF.
Day 3: counting cells
On day 3 an embryo is graded on how many cells it has divided into and how even they look. Around eight cells with even sizing and little fragmentation is the textbook picture. Six is fine. Twelve is not better.
Day 5: the blastocyst grade
By day 5 a healthy embryo has become a blastocyst — a fluid-filled ball with two distinct cell groups. That is what the three-part grade describes.
- The number (1–6): how expanded the blastocyst is, and whether it has begun hatching out of its shell. 3 to 5 is the usual range at transfer.
- The first letter: the inner cell mass — the cells that become the baby. A is tightly packed with many cells; C is few and loose.
- The second letter: the trophectoderm — the cells that become the placenta. Same A to C scale.
So 4AA is a fully expanded blastocyst with strong cells in both groups. 3BB is a slightly earlier blastocyst with fair cells. 5AB has begun to hatch, with a strong inner cell mass and fair placental cells.
How much does grade matter?
On average, top-graded blastocysts implant more often than lower-graded ones. That is why they go first. But the grade cannot see the chromosomes, and chromosomal normality is the main driver of whether a transfer works. A beautiful 4AA can be aneuploid; a modest 3BC can be entirely normal.
Babies are born from BB and BC embryos every day. If your clinic is transferring a lower-graded embryo, it is because it is a viable embryo — not because they have given up on the cycle.
Day 6 and day 7 blastocysts
Some embryos reach blastocyst a day or two late. Their success rates are somewhat lower than day-5 blastocysts but they are far from written off, and freezing a slow developer is standard practice. Ask your clinic how they rank a day-6 embryo against your others rather than assuming it is a spare.
Why the numbers fall at every stage
Attrition is the normal shape of an IVF cycle, and knowing the pattern in advance takes the sting out of the phone calls. As a rough guide: around 80% of mature eggs fertilise, around half of those reach day 5, and of those that do, a proportion will be chromosomally abnormal. Ten eggs collected becoming three frozen blastocysts is not a failure — it is a fairly typical cycle.
What to ask about your own embryos
- How many were mature, and how many fertilised normally?
- How would you rank the embryos we have frozen?
- Which one would you transfer first, and why?
- Do you grade at a fixed time each day, or when each embryo is ready?
Related questions
- Does a lower embryo grade mean a lower-quality baby?
- No. Grade describes appearance before implantation. It has no bearing on the health of a child born from that embryo.
- Why did my clinic transfer a day-3 embryo instead of waiting for day 5?
- Usually because there were few embryos and little to choose between them — in that situation the uterus is a better environment than the incubator.
This page explains how IVF works in the UK and is not medical advice. Your clinic knows your history — take these questions to them.
