IVF embryo grading: What it means for your fertility treatment

During IVF, each embryo is graded to help you and your fertility team decide which one has the best chance of leading to a pregnancy. The question is, what do these embryo grades in IVF mean, and what can they tell you about your chances of success?

This guide explains what IVF embryo grading is, how it works, and what the charts and scores are based on. You will also learn why a high grade doesn’t guarantee success and what other steps you can take to improve your outcomes.

What is embryo grading in IVF?

Embryo grading is a method for evaluating the quality and development potential of embryos created during IVF. The goal is to support decision-making during embryo selection. With grading data, you can identify potentially strong, average, and lower-quality embryos and decide which to transfer, freeze, or have tested for genetic disorders before implantation.

Even though embryos come from the same parents, each one is genetically unique (except in the case of identical twins), which affects cell division and early development.

During the first days after fertilization, embryos undergo rapid mitotic divisions. Tiny differences in timing or accuracy of these divisions can cause some embryos to progress faster or slower than others. Lab conditions, such as slight variations in culture media, temperature, oxygen levels, or handling, also influence the speed of embryo development. As a result, some embryos reach the blastocyst stage (the stage around day 5–6 when the embryo is ready to implant) sooner than others.

It’s normal for embryos from the same IVF cycle to reach developmental milestones at different rates. Embryologists track them individually and may prioritize those that reach certain stages on optimal timelines.

How are embryos graded for IVF?

Embryo grading is done by evaluating the developmental potential of the embryo at different stages. Here is a quick overview of the features embryologists look at when the embryos reach the cleavage stage (day 2 or 3, when cells are actively dividing) and blastocyst stage (day 5 or 6):

Most clinics use similar criteria for evaluation, though terminology and grading systems can differ slightly. Cleavage-stage embryos are often given a numeric grade (say, 1–4 or 5), although some clinics may opt for alphabetical grading.

Blastocyst stage embryos are graded for all three criteria, so a 4AB could mean:

• 4: Fully expanded blastocyst

• A: Excellent ICM

• B: Good TE

Grading embryo quality in IVF: The two-stage assessments

IVF embryo grading typically happens on day 3 and day 5 after fertilization, reflecting different stages of embryo development. Slow-to-grow embryos are monitored through days 6 or 7 before final grading or freezing. While slower growth is linked to higher aneuploidy rates, such embryos can still lead to a successful pregnancy.

Let’s understand what grading means in the two primary stages:

1. Day ~3: The cleavage stage

2. Day ~5: The blastocyst stage

Source: Med Park Hospital

IVF embryo grading on day ~3: Cleavage stage

On day 3 after fertilization, embryos are at the cleavage stage, when their cells are actively dividing, but the overall IVF embryo size remains the same as the unfertilized egg. At this stage, a good embryo typically has 6 to 10 cells, and those with eight or more cells have greater chances of leading to a successful pregnancy.

Embryologists grade day 3 embryos based on:

• Cell number: More cells usually indicate better developmental progress.

• Cell symmetry: Uniformity suggests healthy cell division. 

• Fragmentation: The fewer small bits of cytoplasm that break off during cell division, the better.

In addition to fragmentation, some clinics may note cytoplasmic pitting, which looks like tiny dents inside the cells. Still, experts aren’t sure what this means for embryo health.

Other features observed include compaction, when the cells start to stick closely together as the embryo develops, and vacuoles, which are small fluid-filled pockets within the cells.

What do the cleavage-stage IVF embryo grades look like?

Here’s a sample IVF embryo grading chart  at the cleavage stage:

Note: If your clinic opts for alphabetical grading, the grades can be A–D (or E). These grades may vary by clinic.

Embryos with grades 1 to 2.5 often have a higher potential for developing to the blastocyst stage, but that doesn’t mean grade 3 embryos are always of bad quality. Their appearance may be caused by asynchronous cell division rather than by poor development, so they may still become viable as they progress.

Some clinics also rate embryos only by the amount of fragmentation:

• Good: Less than 10% of fragmentation

• Fair: 10–25% of fragmentation

• Poor: Over 25% of fragmentation

Source: American Surrogacy

IVF embryo grading on day ~5: Blastocyst stage

By day 5, embryos that continue developing reach the blastocyst stage, when they begin to form a fluid-filled cavity and differentiate into two key cell types:

1. The inner cell mass (ICM), which becomes the fetus

2. The trophectoderm (TE), which forms the placenta

At this time, embryos start to outgrow the space inside the zona pellucida or shell (the membrane surrounding the embryo). They expand and stretch this shell, getting ready to burst through it and prepare for the implantation. This is called hatching.

Day 5 embryos are assessed on:

1. Degree of blastocyst expansion, which indicates how much the fluid-filled cavity has expanded. A higher expansion indicates healthy maturation.

2. Quality of the inner cell mass, which indicates the number and organization of cells. A higher quality suggests strong potential for fetal development.

3. Trophectoderm epithelium quality, which indicates the number and organization of outer cells. A well-formed, uniform layer means a higher likelihood of implantation.

What does the blastocyst-stage IVF embryo grade chart show?

Day 5 embryos are graded for all three criteria using the Gardner grading scale. Here’s an IVF embryo quality chart showing what different grades mean:

Some clinics also have grade D for the ICM and TE, suggesting that cells seem degenerate, while others will use grade C if cells seem unhealthy.

IVF success rates by embryo grade

IVF success rates by embryo grade can vary by clinic. The table below shows what success looks like for some cleavage-stage grades:

As for the blastocyst-stage success rates, expanded blastocysts with grades 3 or higher are two times more likely to end in pregnancy than those with grades 1 to 2, which haven’t reached full development. Here are the success rates across some common grades:

It’s also important to consider the exact day embryos reach the blastocyst stage. For some, it’s day 5, while for others, it’s day 6 or 7. Day 5 blastocysts tend to have higher live birth rates, but other embryos can still be viable, especially if they receive high grades.

Although higher-grade embryos have better success rates, lower-grade embryos can and do result in healthy live births. Embryos continue to develop and reorganize throughout the culture process, so their quality can improve.

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What is the best grade of embryos in IVF?

Embryo grading systems can vary slightly between clinics, so what’s considered a top grade for one lab might be labeled differently in another. Some clinics may use the number-letter combinations, while others use descriptive terms like excellent, good, fair, or poor. One of the questions to ask your fertility clinic is how it interprets and communicates embryo quality.

Generally, the best possible blastocyst-stage grades are:

• 4AA

• 5AA

• 6AA

There’s also an active debate over which embryo features better predict implantation success. For instance, a 2021 study found that inner cell mass quality is more closely linked to ongoing pregnancy and live birth. However, other studies suggest that TE quality is a stronger predictor than ICM. A 2025 study found that higher-quality TE (grade A) had more cells, implanted more often, and led to more live births than embryos with lower-grade TE.

In practice, clinics look at a combination of traits, although no single morphological feature guarantees success.

Source: MART Production

Limitations of traditional IVF embryo grading

Embryo grading in IVF reflects what an embryo looks like under the microscope, but it doesn’t tell us everything about its ability to implant or result in a healthy baby. Other factors that influence IVF outcomes include:

• Uterine receptivity and endometrial health: If the uterine lining isn’t thick enough or hormonally primed for implantation, even a top-grade embryo may not implant. Issues like inflammation, polyps, fibroids, or immune responses can impact outcomes.

• Egg and sperm health: Embryo quality in IVF begins at the cellular level. Eggs affected by age or reduced mitochondrial capacity may produce embryos with slower development, even if they initially seem healthy. Sperm DNA fragmentation or poor morphology can also affect embryo viability and success.

• Embryo transfer timing: Transferring the embryo at the time when the uterus is the most receptive is as important as the embryo quality. Some clinics perform an endometrial receptivity analysis to pinpoint the right timing for transfer, especially in cases of recurrent implantation failure.

• Embryo transfer technique: The skill and precision of the embryo transfer procedure can significantly influence success rates.

• Embryo genetics: High-grade embryos can fail to implant for reasons like chromosomal and genetic abnormalities. Preimplantation genetic testing (PGT) can sometimes detect these issues, but it’s also not foolproof.

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How can preimplantation genetic testing help during IVF?

PGT can give you valuable insights into your embryos’ genetic health before transfer. Your options include:

1. PGT-A, which checks for missing or extra chromosomes.

2. PGT-SR, which looks for chromosomal rearrangements (when whole chromosomes or parts of them switch places). The parent carriers often have no symptoms, but the affected embryos can end up with an unbalanced rearrangement and lead to implantation failure, miscarriage, or developmental issues.

3. PGT-M, which screens for specific inherited conditions that may run in your family, such as cystic fibrosis, sickle cell anemia, muscular dystrophy, and Huntington’s disease.

4. PGT-P, which evaluates how combinations of genes influence long-term risk for chronic, adult-onset conditions like:

• Heart disease

• Type 1 and type 2 diabetes

• Certain cancers

The right type of testing depends on your personal fertility and medical history. Nucleus IVF+ can be a perfect fit for your IVF journey if you’re looking for tailored support across transparent consultations, clinical and genetic testing, and embryo selection.

Nucleus IVF+: The clarity that drives better IVF outcomes

Nucleus brings you a network of support tools in an all-inclusive IVF package. You get advanced carrier screening, embryo analysis, and more, so you can move beyond hard-to-comprehend clinical testing and make confident, informed decisions for your pregnancy.

At its core, Nucleus IVF+ is the first IVF program that offers genetic optimization and human-centric care. Instead of piecing together different tests and services, you get concierge-like support — the all-in-one package includes:

You’ll also get ongoing support, covering lab work, unlimited genetic counseling sessions, and 24/7 expert guidance.

Schedule a call with the Nucleus team to explore a suitable package for your needs.

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Featured image source: Yuliia Novikova