Embryo Grading Made Simple: The Gardner Grading Scale
Embryo grading is a very common question that confuses patients and staff alike. If you haven’t already read our page on Embryo Grading and Assessment then we highly recommend you check it out!
Embryo grading is a crucial aspect of in vitro fertilization (IVF), assisting embryologists in evaluating embryo quality and selecting those with the highest potential for successful implantation and pregnancy. Understanding the grading system can help demystify the process for patients undergoing IVF treatments. Most labs will follow the Gardner Grading Scale. This is the gold standard!
Day 3 Embryo Grading (Cleavage Stage)
On the third day post-fertilization, embryos typically consist of 6 to 8 cells. Grading at this stage focuses on several key factors:
Cell Number and Division: An optimal day 3 embryo has about 6 to 8 cells, indicating appropriate development. (ideal: 8 cells by 72 hours)
Cell Symmetry: Embryos with evenly sized cells are preferred, as uniformity suggests healthy development. (evenly sized cells score higher)
Fragmentation: This refers to the presence of cell debris or fragments. Minimal fragmentation is ideal, as excessive fragments can reduce implantation potential. (less than 10% is best)
Grading scales for day 3 embryos can vary among clinics. Some use a numerical system where lower numbers indicate higher quality, while others may reverse this order. Other clinics may even use letter grades with A being the best and D being the worst.
Examples - 8A (best), 4C (worst) or 81 (best), 44 (worst)
Day 5 Embryo Grading (Blastocyst Stage)
By day 5, embryos reach the blastocyst stage, characterized by more complex structures. Grading at this stage typically involves a combination of numbers and letters:
Expansion Grade (Number 1-6): This number indicates the degree of expansion and development of the blastocyst cavity:
1: Blastocoel cavity is less than half the volume of the embryo.
2: Blastocoel cavity is more than half the volume of the embryo.
3: Full blastocyst with the cavity completely filling the embryo.
4: Expanded blastocyst with a cavity larger than the embryo, and the zona pellucida is thinning.
5: Hatching blastocyst, beginning to emerge from the zona pellucida.
6: Fully hatched blastocyst, completely emerged from the zona pellucida.
Inner Cell Mass (ICM) Quality (First Letter A-C): Assesses the group of cells that will develop into the fetus:
A: Many tightly packed cells.
B: Several loosely grouped cells.
C: Very few cells.
Trophectoderm (TE) Quality (Second Letter A-C): Evaluates the cells forming the placenta:
A: Many cells forming a cohesive layer.
B: Few cells forming a loose layer.
C: Very few large cells.
For example, a blastocyst graded as 5AA indicates a hatching blastocyst with both excellent ICM and TE quality.
Importance of Embryo Grading
While embryo grading provides valuable insights into embryo quality, it's important to recognize its limitations:
Subjectivity: Grading is somewhat subjective and can vary between embryologists and clinics. This is arguably the most important thing to remember. Why? Because grading is not equal on all levels. People may see things differently, and given the grading criteria, it is very difficult to be extremely precise. One embryologist may grade different than another or even have variability amongst clinics!
Predictive Value: High-grade embryos have a better chance of implantation, but lower-grade embryos can still result in successful pregnancies. Embryo grading is just a morphological evaluation. It does not actually determine how HEALTHY an embryo is. Unhealthy embryos can look exactly the same as healthy embryos!
Genetic Normalcy: Grading assesses morphological appearance and doesn't guarantee genetic normalcy.
Key Questions Answered
"Is a 3BB better than a 4BC?"
Yes. The first letter (ICM) matters more for implantation, while the second (TE) affects placental development. A 3BB has better ICM quality than 4BC.
"Why did my Day 3 Grade 1 become a Day 5 Grade CC?"
Some embryos arrest development between days 3-5 due to chromosomal abnormalities (even "perfect" early embryos can become poor-quality blastocysts).
"Should we transfer a BC-grade embryo?"
If it's PGT-A tested normal, even BC embryos can succeed (just at lower rates). Untested? Likely better to wait for a higher-grade embryo if available.
What Grading Doesn't Tell You
❌ Chromosomal status (requires PGT-A testing)
❌ True implantation potential (some "perfect" embryos fail)
❌ Future child's health (grading only predicts short-term development)
The Bottom Line
✔ AA/AB/BA/BB grades have best outcomes
✔ BC/CB may work if PGT-A normal
✔ CC embryos rarely succeed (but exceptions exist)
✔ Day 3 grades don't always predict Day 5 quality
Ask Your Embryologist:
"How does your lab's grading compare to national averages?"
"Would you recommend transferring this specific grade?"
Embryo grading is a tool to assist in selecting the embryos most likely to result in a successful pregnancy, but it's not an absolute predictor. Keep this guide in mind next time you receive grades back for your embryos!
“I heard one tech whisper that even a 3BC made it to a live birth last month. Can you believe that used to be a 3BC?”
