Embryo Grading Explained: What Those Numbers Mean

You've just gotten the call from your clinic: "You have three embryos — a 4AA, a 3BB, and a 5BC." What does that even mean? Is 4AA good? Should you be worried about the 5BC?

Embryo grading can feel like a mysterious code, but it's actually a straightforward system embryologists use to assess embryo quality. This guide breaks down exactly what each number and letter means — and why a "lower grade" embryo might still give you a healthy baby.

The Basics: What Embryo Grading Measures

Embryo grading is a visual assessment of an embryo's appearance at a specific moment in time. Embryologists look at:

• Cell number and division: Is the embryo developing at the expected rate?
• Cell appearance: Are the cells uniform and healthy-looking?
• Fragmentation: How much cellular debris is present?
• Structure: Are the key components well-formed?

Grading helps clinics decide which embryos to transfer first and which to freeze. But here's the crucial point: grading is subjective and imperfect. It tells us about appearance, not genetics. A beautiful-looking embryo can be chromosomally abnormal, while a scraggly-looking one can produce a perfectly healthy baby.

Day 3 vs. Day 5 Embryos

Embryos are typically graded at two key stages:

Day 3 (Cleavage Stage)

At Day 3, embryos have 6-10 cells. Grading focuses on:

• Cell number: 7-9 cells is ideal; fewer than 6 suggests slow development
• Fragmentation: Less than 10-15% is considered good
• Cell symmetry: Even-sized cells are preferred

Day 3 grades are often expressed as a single number or letter (Grade 1, Grade A) or a description like "8-cell, Grade 2."

Day 5 (Blastocyst Stage)

By Day 5, embryos have developed into blastocysts with 100+ cells and distinct structures. This is when the familiar number-letter-letter grading (like 4AA) is used.

Most clinics now culture embryos to Day 5 when possible, as it provides more information and better-developed embryos for transfer.

Decoding Blastocyst Grades: The Number-Letter-Letter System

A typical blastocyst grade looks like 4AA or 5BB. Here's what each part means:

What's Considered a "Good" Grade?

But here's what matters more than memorizing this hierarchy: context. A 3BB on Day 5 may have better potential than a 4AA on Day 6, because speed of development also matters.

Success Rates by Embryo Grade

Research shows a correlation between grade and success, but it's not as dramatic as you might think:

*Rates vary by clinic, patient age, and whether PGT-A testing was performed. These are general estimates.

Day 5 vs. Day 6 vs. Day 7 Embryos

Embryos that reach blastocyst stage on Day 5 generally have slightly better outcomes than those reaching it on Day 6 or 7. However:

• Day 6 embryos can and do result in healthy pregnancies
• Research shows Day 7 embryos are worth freezing — about 10% of patients would have no embryos without them
• A "slow" embryo isn't necessarily a bad embryo

Studies have found that a poor-quality Day 5 embryo often has comparable outcomes to a good-quality Day 6 embryo — so day of development and grade both matter.

What Grading Can't Tell You

This is critical to understand: embryo grading cannot detect chromosomal abnormalities. A perfect-looking 5AA embryo can have abnormal chromosomes, while a scraggly 3BC can be genetically normal.

The only way to know if an embryo is chromosomally normal (euploid) is through PGT-A genetic testing. Many clinics now recommend PGT-A, especially for women over 35, as it provides information that grading alone cannot.

Why Clinics Use Different Systems

You may notice different clinics describe grades differently:

• Some use "D" grades in addition to A-C
• Some use numbers instead of letters (4-3-3 instead of 4AA)
• Some use descriptive terms (EB for early blastocyst)
• Some only give a single overall grade

This variation can make it confusing to compare results between clinics. If you're unsure what your grades mean, ask your embryologist — they're happy to explain.

Questions to Ask Your Clinic

• What grading system do you use?
• What are my embryo grades, and what do they mean?
• Which embryo do you recommend transferring first, and why?
• Do you recommend PGT-A testing for my situation?
• What's your clinic's success rate with embryos of similar grades?

The Bottom Line

Embryo grading is a useful tool, but it's not a crystal ball. Higher-graded embryos have statistically better chances, but plenty of "average" embryos become healthy babies. Focus on working with a clinic that communicates clearly, provides good embryo culture conditions, and helps you understand your options.

And if your grades aren't all perfect? That's normal. Most IVF cycles produce a mix of grades — and you only need one good embryo to succeed.

Read more: IVF Over 40 | Two-Week Wait Guide | IVF Cost Guide

Beyond the Grade: What Really Matters

While embryo grading provides useful information, its important not to become overly fixated on the letters and numbers. Many healthy babies have been born from embryos that didnt receive perfect grades. The grading system captures a moment in time and measures visible characteristics that correlate with—but dont guarantee—success. Some embryos that look average on day 5 develop beautifully when given the chance. Conversely, even AA embryos dont result in pregnancy 100% of the time. Your embryologist and doctor use grading as one tool among many to select the best embryos for transfer, but they also consider factors like your history, how the embryos developed, and their overall clinical judgment.