In a surrogacy process you want everyone around you to be focused on maximizing your chances of a successful embryo implantation and pregnancy, and a healthy birth, with as few attempts as possible. One of the parameters that help for a successful transfer is the embryo grading.
The embryos that are frozen before they are transferred to the surrogate are graded according to a visual assessment where an embryologist in a lab determines the quality of the embryo. By using embryos with a higher grading we are helping to maximize our chances of success.
Although the embryo grading system is old, the final grading can change even among embryologists and between the embryo’s stages of development since it is a subjective visual assessment.
With surrogacy, we mainly deal with blastocyst embryos, meaning ones that have reached day five or six of development since fertilization. The embryo’s overall degree of development at this stage is defined on a 1-6 scale: The number indicates the blastocyst's expansion stage — how far it has developed and expanded — not the day since fertilization. An embryo can be on day 5 of development at expansion stage 4, which is why reports often read "Day 5, 4AA". The two numbers are easy to confuse because they live in the same range. At this stage of the embryo you can already decipher two separate groups of cells, ICM (from which the actual fetus develops from) and the trophectoderm cells (that define its external structure). Each of the cell groups is graded separately on a descending scale from A-C according to the number of cells and the cohesion between them.
For example - a 5AA grading means the blastocyst is at expansion stage 5 (hatching out of its shell), with both cell groups at the highest grade - A.
Another example - a 6AB grading means the blastocyst has fully hatched (stage 6), with the ICM at the high grade of A and the outer structure cells not quite as good, graded B.
Good quality embryos will be transferred to the uterus or frozen for future use, while low quality embryos will stop developing at a certain stage.
Statistically, the higher the embryo’s grading, the better its chances of developing into a healthy pregnancy and birth.
Embryos that have undergone advanced tests like PGS or PGT-A and have been found “normal” will usually have the highest gradings, since the test is not only visual.
The full Gardner scale
The number — expansion stage (1–6): 1–2 an early blastocyst, the inner cavity just forming · 3–4 a full, expanding blastocyst · 5 hatching out of its shell · 6 fully hatched.
The first letter — the ICM (the cell group that becomes the fetus): A many tightly packed cells · B fewer cells, medium compaction · C few cells.
The second letter — the trophectoderm (the outer layer that becomes the placenta): A an even layer of many cells · B fewer cells · C few cells, a sparse layer.
Not every clinic counts the same way
Before comparing grades, check the system: some clinics flip the number scale so that 1 is the top (making 1AA the star, not 6AA), and some add letters or marks of their own (a D, pluses and minuses). The exact same embryo can carry a "score" that looks different at two clinics. Always ask your clinic how its system works.
Why comparing grades between couples is pointless
The grade is a subjective visual assessment: it varies between embryologists, between clinics, and even between development stages of the same embryo. It also cannot see things that matter just as much — growth pace and chromosomal status. So "the couple from the group has a 5AA and we only have a 4AB" is a comparison without meaning. Grading exists to help YOUR clinic prioritize YOUR embryos — not to rank families.
