Embryo Grading Chart: What 4BB, 5AA, and the Letters Actually Mean for Your IVF
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Quick answer: An embryo grading chart is the system embryologists use to score a blastocyst on three things: how expanded it is (a number from 1 to 6), the quality of the inner cell mass or ICM (a letter A, B, or C), and the quality of the trophectoderm or TE (a second letter A, B, or C). So a "4BB" is an expanded blastocyst with good ICM and good TE. Higher grades line up with better average pregnancy and live birth rates, but the grade is a snapshot of appearance, not a guarantee — and lower-grade embryos still become healthy babies every day.
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What Is an Embryo Grading Chart — and Why It Matters
You get the call from the lab. Your embryos have a grade. And suddenly you are typing "is a 4BB good enough" into your phone at midnight, scrutinizing every letter like it decides your whole future.
If that is you, take a breath. You are not being dramatic, and you are not alone. After a failed fresh cycle, it is normal to feel overly anxious about every detail. The grade feels like a verdict. It is not.
An embryo grading chart is simply a shared language. It lets embryologists describe what a blastocyst looks like under the microscope on day 5, 6, or 7 of development, using the same terms across clinics. The most widely used version is the Gardner system, which has been the global standard since the late 1990s (Gardner & Schoolcraft, Towards Reproductive Certainty, 1999).
The grade matters because, on average, better-looking embryos implant and lead to live births more often than poorer-looking ones. But "on average" is doing a lot of work in that sentence. The grade describes appearance and developmental stage — not the genetics inside, and not your personal outcome.
Understanding your embryo grading chart is most useful if you are:
About to get your day 5 embryology report and want to read it yourself
Deciding which embryo to transfer first
Trying to make sense of a "lower" grade and whether it is worth transferring or freezing
Comparing a fresh result to a frozen one
Weighing whether to add genetic testing (PGT) on top of grading
How to Read Your Embryo Grading Chart
A blastocyst grade has three parts, in this order: a number, then two letters. Read left to right.
The number (1–6) describes expansion — how much the embryo has grown and how close it is to "hatching" out of its shell. The first letter (A, B, or C) grades the inner cell mass (ICM), the cluster of cells that becomes the baby. The second letter (A, B, or C) grades the trophectoderm (TE), the outer layer that becomes the placenta.
So a 4BB is an expanding blastocyst with a good inner cell mass and good trophectoderm. A 5AA is a hatching blastocyst with top-quality cells in both layers. A 3BC is a full blastocyst with a fair ICM and a weaker TE.
Part of the grade | Score | What it means |
Expansion (the number) | 1 | Early blastocyst — fluid cavity is less than half the embryo |
2 | Blastocyst — cavity is more than half | |
3 | Full blastocyst — cavity fills the embryo | |
4 | Expanded — cavity is larger and the shell has thinned | |
5 | Hatching — cells are starting to break through the shell | |
6 | Hatched — fully out of the shell | |
Inner cell mass / ICM (1st letter — becomes the baby) | A | Many tightly packed cells |
B | Looser group of fewer cells | |
C | Very few cells | |
Trophectoderm / TE (2nd letter — becomes the placenta) | A | Many cells forming a cohesive layer |
B | Fewer cells, looser layer | |
C | Few, uneven cells |
A quick reframe that helps: the expansion number tells you when the embryo got there, and the two letters tell you how it looks now. Neither one tells you what is written in its chromosomes — that is a separate test.
Why Patients Fixate on the Grade
"Is a 4BB good enough?"
This is the single most-searched grade question, and the answer is yes — a 4BB is a solid, very transferable embryo with good odds. Most clinics consider anything at 3BB or better to be good quality. The reason it feels uncertain is that you are comparing it to the "perfect" 5AA you saw in someone else's success story. Your embryo is not competing with the internet. It is competing with the other embryos in your own batch.
"My grade dropped after thawing"
Embryos do not always thaw at exactly the grade they were frozen at, and that is expected. A small dip in expansion after warming is common and usually re-expands within hours. What matters far more for a frozen embryo transfer is whether the embryo survives the warming and re-expands — which the lab checks before transfer.
"All I have is a 'C' — should I give up?"
No. A grade with a C is a lower-probability embryo, not a zero-probability one. We cover the data on this below, but the short version: clinics see healthy babies from C-grade embryos regularly, which is exactly why most labs no longer discard them automatically.
"Two embryos had the same grade and only one worked"
This is the most frustrating part of grading. Two 4BBs can have completely different genetics, and appearance cannot see chromosomes. The grade narrows the odds; it does not remove the role of chance.
How Grading Actually Happens in the Lab
Here is what the process looks like behind the scenes:
Days 1–3: The embryo divides from one cell into many. Some clinics grade day 3 embryos by cell count, but most modern labs wait.
Days 5–7: The embryo reaches the blastocyst stage and gets its number-letter-letter grade. This is the grade on your report.
Who grades it: An embryologist assigns the grade by eye, using the chart above.
The catch: Because a human assigns it visually, two embryologists can grade the same embryo slightly differently. Studies measuring this find good but imperfect agreement between graders (Paternot et al., RBMO, 2015; Devroe et al., BMC Research Notes, 2011).
That last point is the quiet truth of grading: it is a skilled human judgment, which means it carries some subjectivity. That is one reason continuous, technology-assisted monitoring — like the EmbryoScope time-lapse system — has become part of how leading labs add objectivity to the picture.
What the Research Shows About Embryo Grades
Decades of data confirm the core idea: grade tracks with outcome, on average. Good-quality blastocysts implant and lead to live birth more often than poor-quality ones across many large studies (Du et al., Frontiers in Endocrinology, 2022).
The two letters do not carry equal weight. In a series of nearly 11,000 single-blastocyst transfers, live birth was more strongly tied to the inner cell mass (ICM) than to the trophectoderm — embryos with a C-grade ICM had lower odds of live birth than A-grade, but still produced healthy babies (Licciardi et al., Human Reproduction, 2022). The day of development matters too: day 5 blastocysts tend to have higher euploidy and live birth rates than day 6, though day 6 embryos absolutely still work (Irani et al., Frontiers in Endocrinology, 2022).
Factor | What the evidence shows | Source |
Good vs. poor grade | Good-quality blastocysts implant and reach live birth more often | Du et al., Front. Endocrinol., 2022 |
ICM grade (1st letter) | Strongest single predictor of live birth among the letters | Licciardi et al., Hum. Reprod., 2022 |
Trophectoderm grade (2nd letter) | A-grade TE linked to higher live birth than C, even in tested embryos | Zhao et al., Reprod. Sciences, 2025 |
Day 5 vs. Day 6 blastocyst | Day 5 has higher euploidy and live birth on average; Day 6 still succeeds | Irani et al., Front. Endocrinol., 2022 |
Low-grade (C) embryos | Lower odds, but healthy live births occur; should not be auto-discarded | Licciardi et al., Hum. Reprod., 2022 |
Does the Grade Predict a Healthy Baby? The Honest Answer
Here is the question almost no article answers straight: Does a high grade mean a healthy baby, and does a low grade mean it will fail?
The honest answer is that the grade predicts probability, not destiny — for three reasons.
First, the grade cannot see genetics. Appearance and chromosomes are related but not the same. Better-graded blastocysts are more likely to be chromosomally normal — in one study of women under 35, good-quality blastocysts were euploid about 63% of the time versus about 32% for poor-quality ones (Zhao et al., Frontiers in Endocrinology, 2022). But "more likely" is not "always." A good-looking embryo can still be aneuploid, and a modest-looking one can be perfectly normal.
Second, the grade is partly subjective. It depends on who is looking and when. That human variability is real and measurable (Paternot et al., RBMO, 2015).
Third, low grades still make babies. This is the part that gets lost in the panic. Large datasets show C-grade and lower-quality embryos producing healthy live births with normal birth weights, which is precisely why the field has moved away from throwing them out (Licciardi et al., Human Reproduction, 2022).
So if your report is not all A's, you have not lost. You have a probability, and a strong one is worth transferring.
Embryo Grading vs. PGT — How They Work Together
A common confusion is treating the grade and genetic testing as rivals. They answer different questions, and the strongest approach often uses both.
Embryo grading tells you how the embryo looks and how it is developing. It is non-invasive, immediate, and free — but it cannot read chromosomes.
PGT-A (genetic testing) takes a small biopsy of the trophectoderm and checks the chromosome count. It can flag aneuploid embryos the grade would miss — but it adds cost, takes a biopsy, and is not perfect either.
Here is the part people miss: even after you select a genetically normal (euploid) embryo, the grade still matters. Among euploid embryos, better-graded ones implant more often, and A-grade trophectoderm is linked to higher live birth than C-grade (Zhang et al., Journal of Ovarian Research, 2021; Zhao et al., Reproductive Sciences, 2025). Grade and genetics are two lenses on the same embryo — using both gives the clearest picture.
Where AI Goes Beyond the Grade
If the grade is a single photo taken by a human at one moment, the next step is a continuous video scored by software — and that is where Aurea's lab is built differently.
Traditional grading means removing embryos from the incubator for periodic visual checks, which briefly disturbs their environment. Time-lapse monitoring keeps embryos in a stable incubator and films their development continuously, so embryologists can watch how an embryo develops — its morphokinetics — not just how it looks at one timepoint.
On top of that, AI scoring tools like iDAScore and ERICA assign an objective, consistent score to each embryo, helping reduce the human variability that grading alone carries. Early research suggests combining an AI score with traditional morphology can shorten the time it takes to reach pregnancy (AI embryo viability study, Fertility & Reproduction, 2023), and deep-learning models continue to show promise in research settings (Sci. Reports, 2025).
An honest caveat: large reviews have not yet proven that time-lapse or AI selection reliably increases live birth rates compared with skilled conventional grading, and one major randomized trial found no significant live-birth difference (Harper et al., The Lancet [TILT trial], 2024; Cochrane review). What the technology does well today is reduce embryo disturbance and add objectivity and consistency to a process that is otherwise done by eye. At Aurea, that means your grade is one input among several — paired with continuous monitoring and AI scoring inside our AI-integrated IVF lab — rather than a verdict handed down from a single glance.
What to Expect — Timeline, Costs, and Next Steps
Your embryo grades usually arrive in your day 5 or day 6 lab report, a few days after egg retrieval. Here is how to use that report well:
Ask which embryo is being transferred first and why. The lab ranks by grade, development day, and (if done) genetic results.
Ask whether lower-grade embryos are being frozen. They often should be — they still hold real potential.
Ask if time-lapse or AI scoring informed the ranking. If your clinic uses it, the grade is not the only data point.
Do not compare your numbers to strangers online. Your batch, your biology, your odds.
On cost: grading itself is included in a standard IVF cycle — there is no separate "grading fee." Genetic testing (PGT) is an add-on, and conventional IVF in New York commonly runs $15,000–$20,000 before medications. Aurea is built to come in well below that, roughly half the typical NYC price, with less medication waste. You can model your own numbers with our IVF cost calculator before you ever book.
Frequently Asked Questions
What is the best embryo grade on the embryo grading chart?
On the standard chart, the top grade is a fully expanded or hatching blastocyst with A-grade cells in both layers — for example, a 5AA or 4AA. These have the highest average odds, but a 4BB or 3BB is still considered good quality and transfers successfully all the time.
What does a 4BB embryo mean?
A 4BB is an expanded blastocyst (the "4") with a good inner cell mass (first "B") and good trophectoderm (second "B"). It is a strong, transferable embryo with solid pregnancy odds.
What does a 5AA embryo mean?
A 5AA is a hatching blastocyst with top-quality cells in both the inner cell mass and the trophectoderm. It is among the highest grades on the chart.
Can a low-grade embryo become a healthy baby?
Yes. Lower-grade and even C-grade embryos produce healthy live births with normal birth weights in large studies, which is why most labs no longer discard them. A lower grade means lower average odds, not no chance.
Does embryo grade matter if I do PGT genetic testing?
Yes. Even among genetically normal (euploid) embryos, better-graded ones tend to implant more often, and A-grade trophectoderm is linked to higher live birth. Grade and PGT answer different questions and work best together.
Is a day 6 blastocyst worse than a day 5?
Day 5 blastocysts have slightly higher euploidy and live birth rates on average, but day 6 blastocysts regularly lead to healthy pregnancies. A day 6 embryo is not a failed embryo — it simply reached the stage a little later.
The Bottom Line
An embryo grading chart is a shared language for describing how a blastocyst looks and how far it has developed — a number for expansion and two letters for the cells. The grade shifts your odds; it does not decide your outcome. Genetics, the lab's tools, and a measure of luck all share the stage with that letter on your report.
So read your grade, ask good questions, and resist the midnight spiral. A "good enough" embryo is, very often, exactly enough.
If you want a fertility team that pairs continuous time-lapse monitoring and AI scoring with real human guidance — and explains every grade on your report in plain language — book a discovery consultation with Aurea Fertility. We will walk you through your embryos, your options, and your real numbers, without the jargon or the pressure.
Article Sources
Gardner DK, Schoolcraft WB. In vitro culture of human blastocysts. Towards Reproductive Certainty: Fertility and Genetics Beyond, 1999.
Paternot G, et al. A clinically useful simplified blastocyst grading system. Reproductive BioMedicine Online, 2015. Link
Devroe J, et al. Intra- and interobserver analysis in the morphological assessment of early-stage embryos. BMC Research Notes, 2011.
Du QY, et al. Blastocyst morphology and developmental rate vs. euploidy and live birth in PGT-A cycles. Frontiers in Endocrinology, 2022. Link
Licciardi F, et al. Low-grade blastocysts result in healthy live births and should not be discarded. Human Reproduction, 2022. Link
Irani M, et al. Effect of blastocyst morphology and developmental rate on euploidy and live birth. Frontiers in Endocrinology, 2022. Link
Zhao YY, et al. Blastocyst morphology, developmental rate, euploidy and live birth in PGT-A cycles. Reproductive Sciences, 2025. Link
Zhang J, et al. Association between morphologic grading and implantation rate of euploid blastocyst. Journal of Ovarian Research, 2021. Link
AI embryo viability score combined with morphology improves time to pregnancy. Fertility & Reproduction, 2023. Link
Deep-learning model for embryo selection using time-lapse imaging. Scientific Reports, 2025. Link
Time-lapse imaging systems for embryo incubation and selection (TILT). The Lancet, 2024. Link
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