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Embryo grading explained: day 3 and day 5 embryos
Embryo grading is the way the embryology lab describes how an embryo looks under the microscope on a given day. It helps choose which embryo to transfer first. A grade is a guide, not a promise: higher-grade embryos implant more often, but lower-grade embryos can also lead to healthy babies.
Where embryos grow
After the eggs are collected, they are kept in incubators that hold the temperature, gases and humidity steady, much like the body. The embryologist checks the embryos at set times, taking them out only briefly so that they are disturbed as little as possible. Some labs use time-lapse incubators that take pictures of the embryos at regular intervals without moving them.
Day 1: the fertilisation check
The morning after the eggs and sperm are joined, the lab checks which eggs have fertilised normally, seen as two small circles (pronuclei) inside the egg. Normally fertilised eggs are then grown on in the lab.
Day 2 and day 3 embryos
On day 2 an embryo usually has about 4 cells, and on day 3 about 8 cells. The lab looks at:
- The number of cells for that day.
- How even the cells are in size.
- Fragmentation: small pieces broken off the cells. Less fragmentation is better.
Day 5 and day 6 embryos: blastocysts
By day 5 or 6, a growing embryo becomes a blastocyst, with a fluid-filled space, a group of cells that will form the baby (the inner cell mass) and an outer layer that will form the placenta (the trophectoderm). A common grading looks like "4AA" or "3BB":
- The number (1 to 6) shows how far the blastocyst has expanded or hatched.
- The first letter (A, B or C) grades the inner cell mass.
- The second letter (A, B or C) grades the outer layer.
A is the highest grade and C the lowest. Labs may use slightly different systems, so ask your clinic to explain your report.
Reading your embryo report
Your report usually lists numbers at each stage: how many eggs were collected, how many were mature, how many fertilised normally, how many embryos were growing on day 3, and how many reached the blastocyst stage and were transferred or frozen. It is normal for the number to fall at each step:
- Not every egg collected is mature.
- Not every mature egg fertilises.
- Not every fertilised egg keeps growing to day 5 or 6.
The fall is often larger at older ages, because more eggs carry chromosome errors. A drop from egg collection to the final embryo count is expected and, on its own, does not mean that something went wrong.
Day 3 or day 5 transfer?
Growing embryos to day 5 lets the lab see which ones keep developing, and blastocysts are often transferred one at a time. Some embryos stop growing before day 5, usually because of a problem within the embryo itself, such as a chromosome error. When there are only one or two embryos, a day 3 transfer may be chosen. The decision is made for each couple.
How many embryos are transferred?
Transferring one good embryo lowers the chance of twins, which carry more risks for the mother and the babies. The number is decided with your doctor, based on your age, the embryo grades and your history, and within the limits of the Indian ART rules. Any other good-quality embryos can be stored frozen and used in a frozen embryo transfer.
Grading and frozen embryos
Blastocysts are graded before they are frozen. When an embryo is warmed for a frozen transfer, the lab checks that it has survived and watches it re-expand for a few hours before the transfer. Good-quality blastocysts usually survive freezing and warming well.
Embryo testing
Grading only looks at the embryo from the outside. Testing an embryo for genetic problems needs a few cells taken from its outer layer, which is a separate procedure. Under the ART (Regulation) Act 2021 it is allowed only to screen for known genetic or inherited diseases, and your doctor will explain whether it applies in your case.
What grading cannot tell
Grading looks at shape and growth, not at the chromosomes inside. A top-grade embryo can still fail to implant, and an average-grade embryo can still become a healthy pregnancy. Read more about IVF treatment and repeated IVF failure.
Questions you can ask about your embryos
- How many embryos are growing, and what grade is each one?
- Do you suggest transfer on day 3 or day 5 for us, and why?
- How many embryos will be frozen, and how are they stored?
- What grading system does the lab use?
Questions about embryo grading
What does a 4AA blastocyst mean?
The 4 means the blastocyst has expanded, with its fluid-filled space larger than the embryo and its shell thinning, the first A is a high grade for the inner cell mass, and the second A is a high grade for the outer layer.
Can a low-grade embryo give a healthy pregnancy?
Yes. Lower-grade embryos implant less often, but many still lead to healthy babies.
Is a day 5 transfer always better than a day 3 transfer?
Not always. Day 5 culture helps choose embryos that keep growing, but with only one or two embryos a day 3 transfer may be advised.
Why did some of my embryos stop growing?
Some embryos stop developing on their own, often because of chromosome errors within the embryo. This is common and expected.
How many embryos should be transferred?
Often one, to lower the chance of twins. Your doctor decides with you, based on your age, the embryo grades and your history.
Why did I get fewer embryos than eggs?
Not every egg is mature, not every mature egg fertilises, and not every embryo keeps growing. A fall at each step is normal and expected.
Do embryos survive freezing?
Good-quality blastocysts usually survive freezing and warming well. The lab checks each one after warming, before the transfer.
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What you read here explains fertility care in general terms and cannot replace a consultation. Decisions about your own care should be made with a doctor who knows your history and has seen you. We do not carry out or offer sex determination of any kind, which is prohibited by law in India under the PCPNDT Act.