If you have been given a grade like 4AA or 3BB and nobody explained it, this page is for you. An embryo grade is an embryologist's visual assessment of how an embryo looks down a microscope at one moment in time. It is useful, and it is routinely over-read by patients — including, often, in the wrong direction.
Day 3 and day 5 are graded differently
Embryos are assessed at different stages, and the two systems look nothing alike.
On day 3, at the cleavage stage, an embryologist counts the cells, notes how even they are, and estimates how much fragmentation is present. An eight-cell embryo with even cells and little fragmentation is what is being looked for on that day.
On day 5 or 6, the embryo has become a blastocyst and is graded on three things at once, which is where a code like 4AA comes from.
How to read a blastocyst grade
Three parts, in this order:
- The number — how expanded it is. A rising number describes a blastocyst that has expanded further and, at the top of the range, has begun hatching out of its shell. It is a measure of stage, not of quality.
- The first letter — the inner cell mass. This is the group of cells that would become the baby. A is the most tightly packed and well defined.
- The second letter — the trophectoderm. The outer layer of cells that would become the placenta. Again, A describes many cells forming a cohesive layer.
So 4AA describes an expanded blastocyst with a well-defined inner cell mass and a cohesive outer layer. 3BB describes one slightly less expanded, with both cell groups looking less uniform.
What a grade is not
This is the part worth reading twice. A grade is an appearance, not a genetic test. It is assessed by eye, and it does not tell you how many chromosomes the embryo has.
A good-looking embryo can be chromosomally abnormal, and a lower-graded embryo can be chromosomally normal and produce a healthy pregnancy. The only way to assess chromosome count is preimplantation genetic testing, which is a separate test on a biopsy of the embryo.
Grading is also not standardized the way a laboratory blood value is. Different laboratories apply the scales slightly differently, so comparing a grade from one clinic against a grade from another, or against a figure you read in a forum, is not a reliable comparison.
Why a lower grade is not a verdict
Patients routinely arrive distressed about a 3BB. Grades inform which embryo to transfer first when there is a choice; they do not decide whether an embryo is worth transferring.
Which embryo is transferred, and in what order, is decided by your physician with the embryology team, taking the grade together with the day it reached blastocyst, any genetic testing result, and your history. If you want the fuller picture of how many eggs become embryos in the first place, we have written about what happens to ten eggs, and about embryo development day by day.
Where grading happens
Grading is done in our on-site laboratory, which holds accreditation from the College of American Pathologists (CAP), by embryologists based on site. Embryos not transferred can be vitrified and used later in a frozen embryo transfer.
Common questions
What does a 4AA embryo mean?
It describes an expanded blastocyst with a well-defined inner cell mass, the cells that would become the baby, and a cohesive outer layer of cells that would become the placenta. The number describes how far the blastocyst has expanded; the two letters describe those two cell groups.
Is a 3BB embryo bad?
No. A grade describes appearance down a microscope, not chromosomal normality, and embryos graded below the top of the scale regularly produce healthy pregnancies. Grades help decide which embryo to transfer first when there is a choice.
Does a high grade mean the embryo is genetically normal?
No. Grading is a visual assessment and cannot assess chromosome count. A good-looking embryo can be chromosomally abnormal. Only preimplantation genetic testing assesses chromosomes, and it is a separate test.
Can I compare my embryo grade to someone else's?
Not reliably. Laboratories apply the grading scales slightly differently, so a grade from one clinic is not directly comparable to a grade from another.
Who decides which embryo gets transferred?
Your physician, with the embryology team. The decision takes the grade together with the day the embryo reached blastocyst, any genetic testing result, and your clinical history.
Your initial consultation is free for all new patients, and we bill health insurance where you have coverage that applies. Call 212-685-2229 or send us a message to arrange a visit.
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