How Embryologists Identify the 'Champion Embryo': Grading Early Embryos and Blastocysts

You receive your embryology report: "Day 5: One 4AA blastocyst, two 3BB blastocysts." What does this mean for your chances of success?

Here's what matters most: Embryo grading is a helpful tool—but not a crystal ball. A perfectly graded 5AA embryo can be chromosomally abnormal and fail to implant. A lower-graded 3BC embryo can be genetically normal and become a healthy baby. Across thousands of embryos, however, grading patterns do help embryologists make strategic decisions that improve your odds.

This guide explains how grading actually works at our clinic—and what your specific report means for your journey.

How We Monitor Your Embryos: The Embryoscope Advantage

At our clinic, all embryos develop inside Embryoscope time-lapse incubators—special chambers that take photos every 10-20 minutes without ever removing embryos from stable culture conditions

Important distinction: While we monitor development continuously via time-lapse imaging, we intentionally delay formal grading assessments until Day 4 (morula stage) when we perform assisted hatching. This approach:

Minimizes disturbance to developing embryos
Allows embryos to demonstrate their developmental potential before assessment
Aligns with evidence that Day 3 appearance poorly predicts blastocyst quality

We do not perform traditional Day 3 grading assessments. Instead, we let embryos develop uninterrupted until Day 4, when we assess compaction quality during assisted hatching procedures.

Day 5-6: When True Quality Assessment Happens

By Day 5-6, successful embryos transform into blastocysts—complex structures with 100-200 cells organized into distinct compartments

Component	What It Forms	Why It Matters
Inner Cell Mass (ICM)	The future baby	Higher quality = better implantation rates
Trophectoderm (TE)	The placenta	Healthy TE cells must invade the uterine lining for implantation
Blastocoel cavity	Fluid-filled center	Expansion shows developmental progress

We use the Gardner grading system to evaluate these three features:

Format: [Number][Letter][Letter]

Example: 4AA = Expansion stage 4, ICM grade A, TE grade A

Understanding the Numbers (Expansion Stage 1-6)

Stages 1-2: Early blastocyst (cavity forming)
Stage 3: Full blastocyst (fills entire embryo)
Stages 4-5: Expanded/hatching (best implantation potential)
Stage 6: Fully hatched

Understanding the Letters (Quality A-C)

Grade A: Many tightly packed cells (best)
Grade B: Adequate cells, some gaps (good)
Grade C: Few cells, poorly organized (fair)

What the Research Actually Shows: Verified Statistics

Key finding: Even among chromosomally normal (euploid) embryos, morphology matters. Good-grade euploids have 15-20% higher live birth rates than poor-grade euploids.

The Truth About Genetics vs. Appearance

Here's what grading cannot tell you: whether an embryo is chromosomally normal.

Even "excellent" morphology blastocysts (including 4AA/5AA grades) have only a 56% chance of being euploid (chromosomally normal). This means nearly half of top-graded blastocysts carry chromosomal abnormalities that grading cannot detect.

At age 42, the challenge intensifies. Overall euploidy rates decline dramatically with maternal age—only 37% of embryos from women age 42 are euploid, regardless of appearance. When combining advanced maternal age with embryo selection criteria, patients aged 41+ with the best morphology and development still show only 21% euploidy rates.

Chromosomal errors primarily originate during female meiosis (egg formation)—before fertilization occurs. Critically, these genetic errors often don't disrupt early embryo structure: an aneuploid embryo can develop beautiful cellular organization before arresting post-implantation. This explains why morphology selection alone is significantly less effective than PGT-A testing for identifying viable embryos.

Why this matters: A euploid embryo with "fair" grading (3CC) has better odds of live birth (~45%) than an untested "perfect" 5AA embryo (which has ≤56% chance of being euploid). Genetics matter most—but when genetics are equal (all euploid), morphology provides a meaningful tiebreaker for transfer priority.

NGC’s Selection Strategy

When choosing which embryo to transfer first:

Genetics first: Euploid > mosaic > aneuploid
Timing second: Day 5 > Day 6 > Day 7 blastocysts
Morphology third: AA/AB/BA > BB > BC/CB > CC

Example: A Day 5 Grade 4BB euploid embryo should be transferred before a Day 6 Grade 5AA euploid—even though the Day 6 embryo has "better" letters—because development speed matters more than small grading differences.

FAQs

My report shows a 4BB embryo instead of 4AA. Should I worry?

No. The difference between 4BB and 4AA is modest (~5% success rate difference). Both are good-quality embryos with strong potential. Focus on the bigger picture: expansion stage (4 = expanded) and that it reached blastocyst stage on Day 5.

I'm over 40 and all my "perfect" embryos were abnormal. How?

Unfortunately common. At 42+, ~70-80% of embryos are chromosomally abnormal regardless of appearance. Grading evaluates structure ("construction quality"), not genetics ("blueprints"). This is why PGT-A provides valuable information for patients over 38.

Can a "poor" Day 3 embryo become a good blastocyst?

Yes—but we don't grade on Day 3 at our clinic. By culturing embryos uninterrupted to Day 5-6, we let them demonstrate their true potential. Approximately 20-30% of embryos that look modest early can form good-quality blastocysts.

Scientific References

Van den Abbeel E, et al. Association between blastocyst morphology and outcome. Reprod Biomed Online. 2013;27(4):353-361.

Ahlström A, et al. Trophectoderm morphology predicts live birth. Hum Reprod. 2011;26(12):3289-3296.

Kaing A, et al. Earlier blastocyst development predicts euploidy. J Assist Reprod Genet. 2018;35(8):1479-1484.

Racowsky C, et al. Standardization of grading embryo morphology. J Assist Reprod Genet. 2010;27(8):437-439.

Terribile M, et al. Embryo grading in PGT-A testing: does it really matter? Hum Reprod. 2024;39(Suppl 1):deae108.871.

Irani M, et al. Blastocyst development rate influences implantation. Fertil Steril. 2018;110(1):95-102.

Franasiak JM, et al. The nature of aneuploidy with increasing age. Fertil Steril. 2014;101(3):656-663.

Yin B, et al. Comparing Day 5 versus Day 6 euploid blastocyst transfers. J Ovarian Res. 2024;17:25.

Youngster M, et al. Day 3 embryo assessment does not reliably predict blastocyst formation. Zygote. 2024.

Irani M, et al. Morphologic grading of euploid blastocysts influences implantation. Fertil Steril. 2017;107(1):66-71.