Embryo Grading Explained

When your embryos are developing in the lab, each one gets a score - a grading system designed to assess how healthy they look under the microscope. It’s one of the most talked-about (and sometimes misunderstood) parts of IVF. Here’s how it works, what the grades really mean, and why they’re not the final word on your chances.

Why Embryos Are Graded

Embryologists can’t see the genetic code of an embryo just by looking at it, but they can examine its structure, growth rate, and symmetry to get clues about its potential. Grading helps:

• Select which embryo(s) to transfer first

• Decide which are strong enough to freeze

• Provide a baseline for discussing success rates

The Grading System: Day 3 vs Day 5–6

Day 3 Embryos (Cleavage Stage)

• Graded by cell number, symmetry, and fragmentation.

• Ideal cell count is usually 6–10 cells by this point.

• Less fragmentation (cell debris) is generally better.

Day 5–6 Embryos (Blastocyst Stage)

• Graded with a number + two letters system.

  • Number (1–6): Stage of expansion (size and cavity).

  • First letter (A–C): Quality of the inner cell mass (which becomes the baby).

  • Second letter (A–C): Quality of the outer layer, the trophectoderm (which becomes the placenta).

Example: 5AA = a fully expanded blastocyst with top scores for both inner cell mass and outer layer.

Why a “Perfect” Grade Isn’t a Guarantee

Grading reflects appearance, not genetics. A “perfect” embryo could still have chromosomal issues, and a “lower-grade” embryo can still lead to a healthy pregnancy. Other factors matter too:

• Uterine environment

• Embryo genetics (only confirmed by PGT)

• Your overall health and treatment protocol

Balancing Science and Hope

It’s natural to pin your hopes on the best-looking embryo, but remember:

• Grading is a helpful tool, not a crystal ball.

• Many parents have healthy babies from embryos with average or even poor grades.