EMBRYO GRADING

When embryos reach the blastocyst stage (usually day 5–6 after fertilization), embryologists grade them to help predict implantation potential. The system most often used is the Gardner grading system, which has three parts: expansion, inner cells, and trophectoderm. 

1. Expansion Stage (number grade: 1–6)

This describes how much the blastocyst has expanded and whether it’s starting to hatch.

• 1: Early blastocyst (cavity just starting to form)

• 2: Blastocyst (cavity <50% of embryo volume)

• 3: Full blastocyst (cavity completely fills embryo)

• 4: Expanded blastocyst (cavity volume > embryo, zona pellucida thinning)

• 5: Hatching blastocyst (partially escaping the shell)

• 6: Hatched blastocyst (fully out of the shell)

Note: Higher expansion (4–6) generally indicates more developmental progress, which is favorable.

2. Inner Cell Mass (ICM: A, B, C)

This is the cluster of cells that will become the fetus.

• A: Many cells, tightly packed (best)

• B: Several cells, loosely grouped

• C: Few cells, irregular/fragmented

3. Trophectoderm (TE: A, B, C)

This is the layer of cells that forms the placenta.

• A: Many cells forming a cohesive layer (best)

• B: Few cells forming a loose epithelium

• C: Very few large cells, uneven

Example of a grade

A blastocyst will most often be written as the Number, and a grade for each section. Like 3AB, 5CC 

What is an optimal blastocyst grade?

Clinically, 4AA, 5AA, or 6AA are considered the “best quality” and are more likely to implant, but:

• Many BB or even BC/CB embryos have resulted in healthy babies.

• Grading is not the only factor — chromosomal health (euploid vs. aneuploid), maternal age, lab conditions, and uterine environment are all critical.

So, while higher grades (especially with “A” scores) are generally preferred, a “lower” grade embryo can still absolutely lead to a healthy pregnancy.

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