Embryos and embryo grading explained
Short answer: Embryo grading is a way for embryologists to describe how an embryo looks under the microscope, using features like cell number, symmetry and fragmentation on day 3, or the size and structure of the fluid-filled cavity on day 5. Grading helps guide which embryo to transfer first, but it's a description of appearance, not a guarantee of how an embryo will behave once transferred.
Why embryos are graded
When more than one embryo is developing, the embryology team needs a consistent way to compare them and decide which to transfer, and which to freeze. Grading gives a shared language between embryologists, but different clinics can use slightly different grading systems, so numbers aren't always directly comparable between clinics.
Grading a day 3 embryo
On day 3, embryos are usually assessed by the number of cells present, typically expecting somewhere around 6 to 10 cells, along with how even those cells are in size and how much fragmentation, small pieces of broken-off cell material, is visible.
Grading a day 5 blastocyst
By day 5, a good embryo has usually developed into a blastocyst, with a fluid-filled cavity, an inner cell mass that will go on to form the baby, and an outer layer that will form the placenta. Blastocyst grading typically scores the expansion of the cavity and the quality of both cell layers separately.
What the grading letters and numbers mean
Common systems use a number for expansion stage and letters, such as A, B or C, for the inner cell mass and outer layer quality. An embryo might be described, for example, as a 4BB. Your clinic can explain exactly what their own scale means, since this isn't fully standardised.
A lower grade doesn't mean an embryo definitely won't result in a pregnancy, and a top grade doesn't guarantee one will. Grading is one factor clinics use alongside your history and circumstances, not the only thing that decides the outcome.
What happens to embryos that aren't transferred
Suitable embryos that aren't used in a fresh transfer can usually be frozen through a process called vitrification, a fast-freezing technique, for possible use in a future frozen embryo transfer cycle.
PGT-A testing
Some patients choose to have embryos tested for chromosomal abnormalities before transfer, known as PGT-A. This involves removing a small number of cells from the outer layer of a blastocyst. It's an add-on with specific pros, cons and costs worth discussing with your clinic in detail.
Add-ons to ask about carefully
- Assisted hatching, which thins or opens part of the outer shell before transfer.
- Embryo glue, a solution added to the transfer fluid.
- Time-lapse imaging for continuous monitoring without disturbing the embryo.
- PGT-A chromosomal screening.
The HFEA rates the evidence behind fertility add-ons, and it's worth checking their ratings and asking your clinic directly what evidence supports any add-on before agreeing to pay for it.
Questions people ask next
- Does a top-grade embryo guarantee pregnancy?
- No. Grading describes appearance under a microscope and correlates loosely with the chance of implantation, but many other factors, including the womb lining and underlying chromosomal makeup, also matter and aren't fully visible through grading alone.
- Why did my clinic choose to transfer a lower graded embryo?
- Clinics sometimes prioritise other factors, such as the embryo's stage of development matching the ideal transfer day, or freezing the highest graded embryo for a future cycle. It's worth asking your clinic directly about their reasoning for your particular embryos.
- Is day 5 transfer always better than day 3?
- Not necessarily for everyone. Day 5 transfer allows more time to see which embryos develop well, but not all embryos survive to day 5 in the lab even if they might have implanted if transferred earlier. Your clinic will advise based on how many embryos you have.
- How long can embryos stay frozen?
- Storage limits are set by law and clinic policy and can be extended in some circumstances. Ask your clinic about current storage limits and renewal processes, since this has changed in UK law in recent years.
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Read next
Sources
- IVF: what is IVF and how does it work?, HFEA (UK fertility regulator). Link checked 17 September 2026.
- Treatment add-ons with limited evidence, HFEA. Link checked 17 September 2026.
- Guidelines and patient information, ESHRE. Link checked 17 September 2026.
Written by patients, not clinicians. This page has not yet been reviewed by a named fertility professional, so it explains published guidance and points you to the original source instead of giving medical advice. It is not medical advice. Decisions about your treatment belong with your own clinic. Read our editorial policy and corrections policy.