You’ve just received the call from the clinic. The embryologist tells you how many embryos are fertilized, how many are still growing, and then, almost in passing, they give you a grade. A number. A letter. Maybe two letters.
And suddenly, everything else fades away.
You hang up the phone and immediately type it into Google:
“Is 4AA a good embryo?”
“Can a 3BB embryo become a baby?”
“What does CC mean for my chances?”
You find yourself comparing grades with strangers in online forums, calculating odds, trying to decode a system nobody properly explained to you.
At some point of your IVF journey, this will sound familiar. Embryo grading is one of the most emotionally loaded parts of the IVF process, and also one of the most misunderstood. Most patients receive grades without ever being told what they actually mean, what they’re based on, or, most importantly, what they can’t tell us.
This guide is here to change that.
We’re going to walk through embryo grading from Day 1 to Day 5 and beyond, in plain language, with real examples, so that the next time you receive a grade, you can understand it clearly and hold it in its proper context.
What is embryo grading, and why do embryologists use it?
Embryo grading is a visual assessment system. When an embryologist looks at your embryos under the microscope, they observe what they can see: the number of cells, how evenly those cells are dividing, how organized the structure looks, and how far the embryo has developed.
Based on what they see, they assign a grade.
That grade is a standardized way for embryologists to communicate with each other and with your medical team about which embryos look most developed and which ones to prioritize for transfer or freezing.
The key word here is prioritize. Grading is a prioritization tool. It helps embryologists rank embryos in order of transfer when multiple embryos are available. It does not, and this is very important to remember, predict with certainty whether an embryo will implant, whether a pregnancy will happen, or whether a baby will be born healthy.
Grading tells you what an embryo looks like on a specific day, at a specific moment in time. It is a snapshot. And like any snapshot, it captures only what is visible, but not the full story.
How embryo grading works: from day 1 to day 5
There are two distinct grading systems used in IVF, and they apply at different stages of development. Understanding both and knowing which one your clinic is using is the key to making sense of any grade you receive.
Day 1: The Fertilization Check (not a grade yet)
About 16 to 18 hours after eggs are fertilized (either through conventional IVF or ICSI), an embryologist carefully removes the culture dish from the incubator and looks at each egg under the microscope.
What they’re looking for is a sign that fertilization actually happened: two small circular structures called pronuclei, one from the egg and one from the sperm. When both are visible, it confirms that fertilization was successful and that an embryo has begun forming.
This is not a grade in the traditional sense.
It is more of a pass/fail checkpoint:
- Two pronuclei (2PN) = fertilization confirmed, development continues
- Zero pronuclei = fertilization did not occur
- Three or more pronuclei = abnormal fertilization; the embryo cannot be used
This Day 1 check is the very first moment of assessment, and for many patients, it is the first emotional milestone of the IVF journey. Knowing that fertilization happened is a quiet but profound relief.
Day 2 and Day 3: Cleavage Stage Grading
By Day 2, the embryo has started dividing. Embryologists check in on embryos at around Day 3, when healthy embryos are expected to have reached 6 to 8 cells. This is called the cleavage stage.
At this point, there is no formal scoring system with letters. Instead, embryologists observe three things:
1. Cell number
A Day 3 embryo with 6 to 8 cells is considered on track for normal development. Fewer cells may suggest slower development; more than 9 to 10 cells can sometimes indicate too-rapid division, which carries its own considerations.
A study of over 2,000 Day 3 embryo transfer cycles found that live birth rates increased progressively with cell number, from 19% in embryos with 6 or fewer cells, to 38.9% in 8-cell embryos, rising further in embryos with 10 or more cells.
2. Cell symmetry
Are all the cells roughly the same size? Symmetrical cells suggest the embryo is dividing in an organized, healthy way. Uneven cells, where some are noticeably larger or smaller than others, can be a sign of less balanced development.
3. Fragmentation
Fragmentation refers to small fragments of cellular material that break off during division. A small amount of fragmentation is completely normal and is seen in virtually all embryos. It becomes more significant when fragmentation is excessive. This is also graded with a percentage:
- Less than 10% fragmentation: generally considered good
- 10–25%: moderate, still usable
- More than 25%: higher fragmentation, which may affect the embryo’s developmental potential
Some clinics combine these observations into a single score or letter grade for Day 3 embryos. Others describe them separately. If you received a Day 3 grade, ask your embryologist exactly what criteria they used. Different clinics use slightly different systems.
Day 4: The Morula Stage (A Quiet Day in the Lab)
On Day 4, the embryo enters what is called the morula stage. The cells compact together into a solid, mulberry-like cluster.
This is not typically a check day. Embryologists generally leave embryos undisturbed at this point, allowing the transition toward blastocyst to happen as naturally as possible. There is no formal grading on Day 4.
Day 5, 6, and 7: Blastocyst Grading – The Gardner System
This is where the grading system most people have heard about comes in. Embryos that continue developing beyond Day 4 reach the blastocyst stage, and this is where the most detailed, meaningful grading takes place.
The system used worldwide for blastocyst grading is the Gardner Grading System, developed by Dr. David Gardner in the late 1990s. It evaluates three separate components and expresses them as a number followed by two letters, for example, 4AA or 3BC.
Here is exactly what each part of that grade means.
Part 1: The Expansion Number (1 to 6)
The first part of a blastocyst grade is a number from 1 to 6.
This describes how much the blastocyst has expanded, essentially, how far along in its development it is at the time of assessment.
| Number | What It Means |
| 1 | Early blastocyst: the fluid-filled cavity is just beginning to form |
| 2 | Blastocyst: the cavity is less than half the volume of the embryo |
| 3 | Full blastocyst: the cavity fills more than half the embryo |
| 4 | Expanded blastocyst: the cavity fills the entire embryo, the outer shell is thinning |
| 5 | Hatching blastocyst: the embryo is beginning to break out of its outer shell |
| 6 | Hatched blastocyst: the embryo has fully escaped its outer shell |
Grades 4 and 5 are generally considered the most desirable for transfer, as the embryo has demonstrated strong expansion and is well-positioned for implantation.
A Grade 6 (hatched) blastocyst is fully expanded and has already started the process of escaping its shell, which is a sign of vitality, though it requires careful handling.
Grades 1 and 2 represent earlier development. An embryo that is still a Grade 1 or 2 on Day 5 may simply be a slower developer. Some embryos reach full blastocyst stage on Day 6 or even Day 7, and that is not automatically a reason for concern.
Part 2: The Inner Cell Mass Grade (A, B, C, or D)
The second part of the grade is a letter: A, B, C, or D. This grades the Inner Cell Mass, often abbreviated as ICM.
The Inner Cell Mass is the cluster of cells inside the blastocyst that will eventually develop into the baby. This is why many patients focus most intensely on this part of the grade.
| Letter | What It Means |
| A | Many tightly packed, well-organized cells: considered excellent |
| B | Fewer cells, loosely grouped: considered good |
| C | Very few cells, or poorly organized: considered fair |
| D | Degenerate or very sparse cells: considered poor |
An A grade ICM is tightly packed and clearly defined.
A B grade ICM still has developmental potential but is less organized.
C and D grades suggest the inner cell mass is less developed, though, as we will discuss shortly, this does not automatically mean the embryo cannot result in a healthy pregnancy.
Part 3: The Trophectoderm Grade (A, B, C, or D)
The third part of the grade is another letter, using the same A to D scale. This one grades the trophectoderm, abbreviated as TE.
The trophectoderm is the outer layer of cells surrounding the blastocyst, the cells that will eventually form the placenta. It is what allows the embryo to implant into the uterine lining.
| Letter | What It Means |
| A | Many cells forming a cohesive, well-organized layer: considered excellent |
| B | Fewer cells, some irregularity: considered good |
| C | Very few cells, or loosely scattered: considered fair |
| D | Severely sparse or fragmented: considered poor |
Trophectoderm grade is very important.
The trophectoderm cells are what actually make implantation happen.
When an embryo arrives in the uterus, it doesn’t implant through its inner cell mass. It implants through the trophectoderm. These outer cells are the ones that physically reach out, attach to the uterine lining, and begin the process of burrowing in. They are the embryo’s only point of contact with the mother’s body at that stage.
They also go on to form the placenta, which means they are responsible for:
- Delivering oxygen and nutrients to the developing baby
- Removing waste
- Producing pregnancy hormones like hCG
- Regulating the immune relationship between mother and embryo
So if the trophectoderm is sparse, disorganized, or poorly structured, the embryo may struggle to implant even if its inner cell mass looks perfect. A beautiful ICM grade means very little if the embryo can’t successfully attach to the uterine wall in the first place.
One meta-analysis of 33 studies found the trophectoderm grade had the strongest association with live birth. And now we know it’s because it is the most functionally critical layer at the exact moment of implantation.
How to Read Your Full Embryo Grade: A Simple Decoder
Now that you understand each component, let’s put it all together. When your clinic gives you a grade like 4AA or 3BC, here is exactly how to read it:
Grade = Expansion Number + ICM Letter + Trophectoderm Letter
Here are some real examples with plain English translations:
| Grade | What It Means in Plain Language |
| 5AA | Hatching blastocyst with excellent inner cell mass and excellent trophectoderm. Top quality. |
| 4AA | Well-expanded blastocyst, excellent in both ICM and trophectoderm. Considered ideal for transfer. |
| 4AB | Well-expanded, excellent ICM, good trophectoderm. Still very strong. |
| 4BB | Well-expanded, good ICM and trophectoderm. A solid, usable embryo. |
| 3BC | Full blastocyst, good ICM, fair trophectoderm. Moderate quality but still worth transferring. |
| 2CC | Early blastocyst, fair ICM and trophectoderm. Lower grade but not without hope. |
| 6AA | Fully hatched, excellent quality. Very advanced & requires careful handling. |
One thing worth knowing: not all clinics use exactly the same system. Some use A, B, C only (no D). Others add a D or even E grade. Some clinics give blastocysts a single letter rather than a number and two letters.
This inconsistency is a genuine limitation of embryo grading and one reason you should always ask your clinic to explain their specific system, rather than comparing your grade directly to someone else’s at a different clinic.
Embryo Grading and IVF Success Rates: What the Research Actually Shows
This is the section most people are searching for when they look up embryo grades. And it deserves an honest, balanced answer, not just reassurance, but real data.
Higher grades do correlate with better outcomes
The research is clear that embryo grade is meaningfully connected to IVF success rates. In Dr. Gardner’s own foundational study, when patients received two top-scoring blastocysts, implantation and pregnancy rates reached 70% and 87% respectively. More broadly, AA-grade blastocysts are associated with pregnancy rates of around 60 to 70%.
The expansion number also matters. Research has shown that fully expanded blastocysts (grades 4 and 5) are approximately twice as likely to result in a clinical pregnancy as early-stage blastocysts (grades 1 and 2).
Day 5 blastocysts generally outperform Day 3 embryos, but the full picture is more nuanced
Studies consistently show that Day 5 blastocyst transfers result in higher implantation rates than Day 3 cleavage-stage transfers. One well-cited study found 161% increased odds of live birth after transfer of a good-quality Day 5 embryo compared to a good-quality Day 3 embryo.
Day 5 transfers also align more closely with natural biology: in a natural conception, the embryo arrives in the uterus at around Day 5 to 7 after fertilization, so a blastocyst transfer mirrors the timing your body already expects.
However, a 2024 randomized controlled trial published in the BMJ found no significant difference in cumulative live birth rates (meaning total success across all transfers from one egg retrieval) between Day 3 and Day 5 approaches.
Some embryos that stop developing in the lab before reaching Day 5 might have thrived if transferred to the uterus earlier. Why? Well for start, the laboratory incubator, even with the most advanced technology available, is not the same as the uterine environment.
This means that Day 3 transfer is not a lesser option. It is a different strategy, and for some patients with fewer embryos or a history of poor blastocyst development, it may actually be the wiser choice.
Lower grades still produce babies
This is perhaps the most important data point of all, and the one least often discussed in online forums where patients compare grades.
Studies confirm that embryos of all quality levels have resulted in live births. Even CC-grade blastocysts, embryos with Grade 2 expansion, and embryos that only reach blastocyst stage on Day 7 have gone on to become healthy children. Yes, grading predicts statistical likelihood, but it does not determine individual outcome.
What Your Embryo Grade Does NOT Tell You
This is where we need to be very direct, because misunderstanding this is the source of enormous unnecessary anxiety for patients.
Your embryo’s grade does not tell you:
Whether your embryo is chromosomally normal. Why?
A perfect 5AA blastocyst can carry a chromosomal abnormality. A 3BC blastocyst can be genetically completely normal. Appearance and genetics are separate things entirely. The only way to know an embryo’s chromosomal status is through genetic testing (PGT).
Whether your embryo will implant. Why?
Implantation depends on many factors beyond embryo grade: the receptivity of your uterine lining, hormonal levels, immune factors, timing, and variables that science still does not fully understand. Some of the most beautiful embryos fail to implant, while less impressive-looking ones attach and grow.
Whether your cycle will succeed. Why?
A single grade is a snapshot of one moment. It does not account for your age, your medical history, your uterine environment, or the countless biological variables that influence whether a pregnancy takes hold.
How your embryo compares to embryos at other clinics. Why?
Because grading is subjective and each lab applies its own standards, a 4AB at one clinic might be graded a 4BB at another. Comparing grades across clinics, or against other patients online, is rarely meaningful.
As the embryologists in the Behind the Lab Doors: What Really Happens During IVF course explain it: grading is not a crystal ball. It is a guide! It is a structured language that helps embryologists make thoughtful decisions and communicate clearly. It is always considered alongside genetics, medical context, and the unique story of each IVF cycle.
Questions People Ask That Deserve Real Answers
“My embryo is a Day 6 blastocyst. Does that mean it’s worse?”
Not necessarily. Some embryos simply develop on a slightly slower schedule. A Day 6 blastocyst has taken one additional day to reach full blastocyst stage, and statistically, Day 6 blastocysts do have slightly lower implantation rates on average than Day 5, but they absolutely can and do result in healthy pregnancies. Many fertility specialists consider Day 6 blastocysts strong candidates for transfer, particularly when they carry a good expansion and letter grades.
“My embryo changed grades between Day 3 and Day 5. How?”
This is completely normal and is actually a sign that your embryologist is doing their job well. Embryos are constantly changing. An embryo that looked average on Day 3 may develop beautifully into a strong blastocyst. The grade at one developmental stage doesn’t lock in the grade at the next.
“Which matters more – the ICM grade or the trophectoderm grade?”
Both matter, but research suggests the trophectoderm grade may be particularly significant, since those cells form the placenta and directly enable implantation. A meta-analysis of 33 studies found the trophectoderm A-grade had the strongest association with live birth of all three grading components. That said, all three components are evaluated together, and no single letter is the deciding factor.
“Should I be worried if I only have lower-grade embryos?”
It is natural to feel anxious. But lower-grade embryos are transferred every day, and they result in pregnancies every day. If they are your only embryos, they are absolutely worth transferring. Have an honest conversation with your medical team about the specific grades, what they mean in your particular case, and what the transfer plan looks like. Your doctor’s recommendation is based on far more than the grade alone.
“Does embryo grading predict the health of the baby?”
No. Embryo grade reflects morphology – how the embryo looks at a cellular level. It does not predict the health, intelligence, or any other characteristic of the child that may result. A lower-grade embryo that implants and develops normally will become an entirely healthy baby.
Behind the Grade: What Embryologists Wish You Knew
After years of looking at embryos under the microscope, embryologists develop a perspective that most patients never get to hear. And it is worth sharing.
Grading is a tool they use to prioritize, not to pass judgment. When an embryologist assigns a grade, they are not saying this embryo is good or bad, worthy or unworthy. They are describing what they observe, using a shared language designed to help make decisions in an inherently uncertain process.
They also know, better than anyone, that embryos don’t read their grades. The embryo that became your child did not consult its score before implanting. Biology operates beyond the vocabulary we use to describe it.
What embryologists focus on, the real work behind the grade, is protecting each embryo with the same level of care regardless of its score. Temperature stability, minimal disturbance, identity verification, environmental controls: every embryo in the lab receives the same protection. The grade influences the sequence of decisions. It does not change the level of care.
Want to Understand Everything That Happens in the IVF Lab?
Embryo grading is just one piece of a much larger picture. And if reading this guide has made you curious about everything else that happens behind the lab doors, like what fertilization actually looks like, how embryos are protected, what a biopsy involves, how freezing works, and what embryologists experience as they care for the earliest moments of life – there is a Family By Choice course built exactly for that.
Behind the Lab Doors: What Really Happens During IVF is an expert-led online course created by Family By Choice, designed specifically for intended parents who want to understand the lab side of IVF, without medical jargon, without sales pressure, and without judgment.
The course covers all 8 stages of the laboratory journey:
- The IVF lab itself: who embryologists are, how they train, and what their days look like
- Retrieval day: what happens the moment eggs leave the operating room
- Embryo development and grading: exactly what was covered in this article, and much more, in video with real professional insight
- Safety and identity protection: how labs ensure your eggs, sperm, and embryos are always yours
- Embryo biopsy and genetic testing (PGT)
- Day 3 vs Day 5 transfer decisions
- Freezing, storage, and long-term care
- The emotional and human side of embryology, including the myths and misconceptions that cause patients the most unnecessary worry
The course is taught by Daria Harvey and Olga Pysana.
It is available at beginner level, in 6 languages (English, French, German, Spanish, Serbian, and Croatian), through the Family By Choice Learning Platform app.
Ready to Go Behind the Lab Doors?
If you are preparing for IVF, currently in treatment, or trying to make sense of a cycle that didn’t go as expected, understanding the lab is one of the most empowering things you can do for yourself.
Knowledge doesn’t remove uncertainty from IVF, but it transforms your relationship with it. Instead of waiting anxiously for a grade and Googling what it means, you’ll know the language, understand the process, and be able to ask the right questions at the right moments.
Explore the Behind the Lab Doors course on the Family By Choice Learning Platform or check your Family By Choice membership – the course may already be waiting for you.
This article is written for educational purposes and does not replace personalized medical advice. Always discuss your specific embryo grades and treatment plan with your fertility specialist.
