Embryo Grading Explained: Understanding Your Embryo Quality Report

Learn how embryos are graded using the Gardner scale. Understand what AA, AB, BB grades mean and how grading affects transfer decisions.

Embryo grading is a systematic method embryologists use to assess the quality and developmental potential of embryos created through IVF. While grades provide valuable guidance for transfer decisions, understanding what they actually measure—and what they don't—empowers you to have informed discussions with your medical team and maintain realistic expectations throughout your journey.

The Gardner Grading System Explained

Developed by Dr. David Gardner in the 1990s, this is the most widely used blastocyst grading system worldwide. A blastocyst grade consists of three components: a number (1-6) indicating expansion stage, followed by two letters (A, B, or C) rating the inner cell mass and trophectoderm respectively.

  • Stage 1: Early blastocyst, cavity less than half the embryo volume
  • Stage 2: Blastocyst, cavity more than half the embryo volume
  • Stage 3: Full blastocyst, cavity completely fills the embryo
  • Stage 4: Expanded blastocyst, cavity larger than embryo, thinning zona
  • Stage 5: Hatching blastocyst, beginning to emerge from zona
  • Stage 6: Hatched blastocyst, completely out of zona pellucida

The first letter grades the Inner Cell Mass (ICM)—the cells that become the baby. The second letter grades the Trophectoderm (TE)—the cells that become the placenta.

Understanding Letter Grades

The letter grades assess the quality and organization of the two critical cell populations within the blastocyst. These assessments are somewhat subjective and can vary between embryologists.

  • Grade A (ICM): Many tightly-packed cells, well-organized
  • Grade B (ICM): Several loosely-grouped cells
  • Grade C (ICM): Very few cells visible
  • Grade A (TE): Many cells forming cohesive epithelium
  • Grade B (TE): Few cells, loose arrangement
  • Grade C (TE): Very few large cells

Common Grades and What They Mean

Understanding typical grade combinations helps you interpret your embryo report:

  • 5AA/6AA: Excellent quality, highest implantation potential (50-65% per transfer)
  • 4AA/5AB/5BA: Very good quality, strong candidates for transfer
  • 4BB/5BB: Good quality, commonly transferred with good success
  • 4BC/4CB: Fair quality, can still result in healthy pregnancies
  • 3CC/4CC: Lower quality but not zero chance—healthy babies are born from these

What Grades Don't Tell You

Morphological grading has important limitations. It cannot detect chromosomal abnormalities (aneuploidy), which require PGT-A testing. It's also a snapshot assessment that doesn't capture the embryo's dynamic development over time. Research shows that lower-grade embryos can produce healthy babies, while some high-grade embryos fail to implant for reasons we can't see.

A beautiful 5AA embryo can be chromosomally abnormal, while a modest 4BB embryo can be chromosomally normal and result in a healthy pregnancy. Grade ≠ genetics.

Grades and Implantation Success Rates

While higher grades correlate with better outcomes, the relationship isn't absolute. Studies show implantation rates approximately: AA embryos 50-65%, BB embryos 40-55%, BC/CB embryos 30-45%, CC embryos 20-35%. These rates vary significantly by maternal age and whether PGT-A testing was performed.

Key takeaways

  • Blastocyst grades combine expansion stage (1-6) with ICM and trophectoderm quality (A-C each)
  • The first letter (ICM) represents cells that become the baby
  • The second letter (TE) represents cells that become the placenta
  • Grading is subjective and can vary between embryologists
  • Morphology cannot detect chromosomal abnormalities—only PGT-A can
  • Lower-grade embryos can absolutely result in healthy pregnancies
  • Your clinic's specific success rates matter more than general statistics

Frequently asked questions

Should I only transfer AA embryos?

Not necessarily. If you have AA embryos available, they're typically prioritized. However, if your best embryo is a BB or BC, don't lose hope—many healthy babies are born from these grades. The decision should factor in your age, embryo count, and whether PGT-A results are available.

Why did my embryo grade change from day 5 to day 6?

Embryos continue developing after initial assessment. Some embryos graded on day 5 may look better or worse on day 6 as they continue expanding or begin hatching. This is normal and doesn't necessarily indicate a problem.

Can embryos be re-graded after thawing?

Yes. Frozen embryos are typically re-assessed after thawing. Most embryos (95%+) survive vitrification well and may even look slightly different than before freezing—this is normal and expected.

My clinic uses a different grading system. How do I compare?

Some clinics use SART, Istanbul Consensus, or proprietary systems. Ask your embryologist to explain their specific scale and what grades they consider good candidates for transfer. The principles (assessing expansion and cell quality) are similar across systems.