Bottom Line Up Front
Embryo grading uses the Gardner scale to describe blastocyst quality with a number (expansion stage 1–6) and two letters (inner cell mass grade + trophectoderm grade, each A–C). A “5AA” is a fully hatched blastocyst with top-quality inner cell mass and trophectoderm. But a “4BB” or even “3BC” can absolutely result in a healthy baby. Grading is a rough indicator of implantation potential, not a definitive predictor.
If you have been through an IVF cycle, you have probably received an embryology report with grades like “5AA,” “4BB,” or “3BC” and wondered what they mean. The grading system used for blastocysts (day 5–6 embryos) was developed by Dr. David Gardner and is the global standard. Understanding it helps you interpret your results and have informed conversations with your fertility team.
The Three Components of a Blastocyst Grade
The Number: Expansion Stage (1–6)
The first character indicates how expanded the blastocyst is. Stage 1 is an early blastocyst with a small cavity. Stage 2 has a cavity filling more than half the embryo. Stage 3 is a full blastocyst with a complete cavity. Stage 4 is expanded, with the cavity larger and the outer shell (zona pellucida) thinning. Stage 5 is hatching, with the embryo beginning to break through the zona. Stage 6 is fully hatched. Higher numbers indicate more advanced development and generally correlate with higher viability, but a Stage 3 transferred on day 5 is perfectly normal and healthy.
The First Letter: Inner Cell Mass (ICM) Grade
The ICM is the clump of cells inside the blastocyst that will become the baby. Grade A means many tightly packed cells, well-organized. Grade B means fewer cells, slightly less compact. Grade C means very few cells. Grade A ICM is associated with higher implantation rates, but Grade B embryos regularly result in healthy pregnancies.
The Second Letter: Trophectoderm (TE) Grade
The trophectoderm is the outer ring of cells that will become the placenta and supporting structures. Grade A is a cohesive layer with many cells. Grade B has fewer cells, slightly irregular. Grade C has very few cells, scattered. Like ICM grading, Grade A is ideal but Grade B is excellent and Grade C embryos can and do succeed.
What the Grades Mean for Your Chances
| Grade | Description | Approximate Implantation Rate |
|---|---|---|
| AA (excellent) | Top quality in both ICM and TE | 50–65% |
| AB or BA (very good) | Excellent in one, good in the other | 45–55% |
| BB (good) | Good quality in both | 35–50% |
| BC or CB (fair) | Good in one, lower in the other | 25–40% |
| CC (poor) | Lower quality in both | 10–25% |
Grading Is Not Destiny
These are population averages. Plenty of “BB” embryos become healthy babies, and some “AA” embryos do not implant. Morphological grading is a visual assessment under a microscope. It does not evaluate the genetics of the embryo. A gorgeous-looking AA embryo can be chromosomally abnormal, and a modest-looking BB can be perfectly healthy. That is why PGT-A (chromosomal screening) provides additional, independent information that morphology alone cannot.
AI-Assisted Embryo Selection
Traditional grading is subjective — different embryologists may assign different grades to the same embryo. AI-based tools are increasingly being used to supplement human grading. These systems analyze time-lapse images of embryo development (how the cells divided, when key milestones were reached, morphological features at multiple time points) and assign a score correlated with implantation probability. Some Colombian clinics have adopted time-lapse incubators and AI-assisted selection tools, adding an objective data layer to the subjective art of embryo grading.
Questions About Your Embryo Report?
A Colombian fertility specialist can review your embryology report and help you understand what your grades mean for your specific treatment plan.
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