How does embryo grading work?

Today’s blog was written by our case manager, Lindsey B.

While there a few different grading scales used for assessing embryo quality, the most commonly used is called the Gardner grading scale.

Embryos are first given a number from 1-6. A common misconception is that this number indicates the day the embryo was frozen, but the number is actually a reflection of the amount of expansion the embryo displays! Below is a quick rundown:

1. Early blastocyst, with the inner cavity less than half the volume of the embryo
2. Blastocyst with inner cavity half or greater than half the volume of the embryo
3. Full blastocyst. The inner cavity should be the full volume of the embryo at this point!
4. Expanded blastocyst. The outer zona should be thinner as the embryo expands
5. Hatching blastocyst, with the trophectoderm cells beginning to hatch out of the zona
6. Hatched blastocyst that has fully hatched out of the zona

Next, the inner cell mass (the part of the embryo that eventually becomes the fetus!) is given a grade from A-C.

• A. Many tightly packed, compact cells
• B. Fewer and more loosely grouped cells
• C. Very few cells

Finally, the trophectoderm (the cells all around the outside of the embryo that eventually become the placenta) is also given a grade from A-C.

Together, these grades provide an indication of the overall quality of the embryo (for example, 5AA) that embryologists and providers can use when determining which embryo to transfer! Embryo grades can change as they continue to develop, but they are given a final grade prior to being frozen or transferred. All labs have different requirements, but most prefer to transfer expanded embryos (4 or greater) with at least a B grade for both the inner cell mass and trophectoderm!