Understanding Embryo Grading: What It Means, Why It Matters, and How to Evaluate "Good" Grades

Embryo grading plays a critical role in assisted reproductive technology (ART), providing valuable insights into the quality of embryos and their potential of developing into a healthy pregnancy. But — and I speak from personal experience — embryo grading can also be one of the most complicated aspects of ART (an already complex process!) to navigate. In my case, I actually had one fertility clinic — where I did my retrieval cycles — disagree with my new clinic about which embryo was the best quality to transfer. When transfer day came and the embryologist read aloud which embryo they’d be using, imagine my surprise when it was a different embryo than the one I expected. 

Thankfully, my (very patient) reproductive endocrinologist walked me through my clinic’s data on various embryo grades and the success rates of each. And luckily, this embryo transfer resulted in the healthy birth of my son. But, since we all don’t know what we don’t know, the goal of this article is to ensure that you are armed with all the facts about embryo grading. We’ll discuss how different embryo grades influence the chances of success in ART procedures, including IVF. That way, you can ask the right questions, make more informed decisions, and maximize your prospects of achieving a successful pregnancy — with donor eggs or your own. Let’s jump in. 

What is embryo grading?

Embryo grading involves the visual assessment of morphological characteristics of an embryo by a trained embryologist, to evaluate the quality of embryos and their chances of developmental success into a healthy pregnancy. In other words, embryo grading is important because it can give you an idea of your odds of success prior to transferring that embryo. It can also provide insight into whether or not another IVF cycle might be beneficial, with or without adjustments to the treatment protocol.  

During the embryo grading process, embryologists evaluate factors such as cell number, cell symmetry, fragmentation, and the appearance of the zona pellucida. These assessments are typically conducted at the embryo’s cleavage stage (day 2 or 3 of development) and the blastocyst stage (day 5 or 6 of development) following fertilization, for embryos that have made it to this point. 

Understanding different embryo grades and their significance

Embryo grading systems provide a framework for embryologists to assign ratings based on specific morphological features. And while these systems are standardized, the grading system used can vary depending on the clinic. Embryo grading systems are equally applicable in donor egg IVF cycles, enabling the evaluation of embryos derived from donor eggs. 

Some commonly used embryo grading systems include: 

Gardner's blastocyst grading system 

The Gardner embryo grading system (sometimes referred to as the Gardner and Schoolcraft system) is widely used, and takes into account three main factors: expansion grade, inner cell mass (ICM) grade, and trophectoderm (TE) grade. Here's a breakdown of each component:

Expansion grade 

The expansion grade refers to how much the embryo has grown and expanded in the culture medium. It is usually assessed on a scale from 1 to 6. Here's what each grade means:

• Grade 1: The embryo is not expanded or only slightly expanded.
• Grade 2: The embryo has started to expand, with a small fluid-filled cavity called a blastocoel.
• Grade 3: The embryo has further expanded, and the blastocoel is larger.
• Grade 4: The embryo is even more expanded, with a significant blastocoel.
• Grade 5: The embryo has expanded further, with the blastocoel filling most of the embryo.
• Grade 6: The embryo has fully expanded, with a large blastocoel and thinning of the outer layer.

A higher expansion grade generally indicates better developmental potential.

Inner Cell Mass (ICM) grade

The ICM is a cluster of cells inside the embryo that will eventually develop into the fetus. It is graded based on its appearance and overall quality. Here's a simplified breakdown of ICM grades:

• Grade A: The ICM is tightly packed, with many cells and high density.
• Grade B: The ICM is looser or slightly less dense than Grade A, but still considered good quality.
• Grade C: The ICM is fragmented or has fewer cells, indicating lower quality.

Embryos with a higher ICM grade have a higher likelihood of successful implantation and pregnancy.

Trophectoderm (TE) grade

The trophectoderm is the outer layer of cells that will develop into the placenta. It is also graded based on its appearance and overall quality. Here's a simplified breakdown of TE grades:

• Grade A: The TE is tightly packed, forming a cohesive layer.
• Grade B: The TE is slightly looser or has minor irregularities but is still considered good quality.
• Grade C: The TE is fragmented or has more significant irregularities, indicating lower quality.

Embryos with a higher TE grade generally have a higher chance of successful implantation and pregnancy.

Combining the expansion grade, ICM grade, and TE grade provides an overall assessment of the embryo's quality and potential for successful implantation and pregnancy. However, the grade of the embryo is just one piece of the puzzle. The success of embryo transfer depends on various individual factors, including your age, overall fertility health, uterine environment, and the skill of the doctor.

Society for Assisted Reproductive Technology (SART) grading system

The SART embryo grading system focuses on the evaluation of cleavage-stage embryos (day 2 or 3). It takes into account parameters such as the number and symmetry of blastomeres, the presence of fragmentation, and the appearance of the cytoplasm. Embryos are generally graded on a scale from 1 to 4:

• Grade 1: These embryos are considered of excellent quality. They have equal-sized and symmetrical blastomeres (cells) without any fragmentation. These embryos have a higher likelihood of successful implantation and pregnancy compared to lower-graded embryos.
• Grade 2: These embryos are still of good quality but may show minor asymmetry or minimal fragmentation. Although they may have slight imperfections, they still have a reasonable chance of successful implantation and pregnancy.
• Grade 3: These embryos have moderate asymmetry or fragmentation. The presence of moderate irregularities may slightly decrease their chances of successful implantation and pregnancy compared to higher-graded embryos.
• Grade 4: These embryos exhibit significant asymmetry or fragmentation. These embryos have more substantial irregularities, significantly reducing their chances of successful implantation and pregnancy compared to higher-graded embryos.

Cummins' embryo grading system:

The Cummins' grading system is another method used to assess cleavage-stage embryos. It is quite similar to the SART system, but while the SART system primarily focuses on the symmetry of blastomeres and the degree of fragmentation, the Cummins system emphasizes the size and fragmentation of those blastomeres. 

Unlike the SART grading system, through the Cummins embryo grading system, embryos are actually scored on a scale from 1 to 5 (vs. 4):

• Grade 1: Embryos that receive a Grade 1 are considered of excellent quality. They have blastomeres (cells) that are of equal size and show no fragmentation. Grade 1 embryos have a higher chance of successful implantation and pregnancy compared to lower-graded embryos.
• Grade 2: Grade 2 embryos are still considered good quality, but they may have minor imperfections. These embryos might have some minor asymmetry or minimal fragmentation, meaning that the blastomeres may not be perfectly equal in size, and there may be some small fragments. Grade 2 embryos still have a reasonable chance of successful implantation and pregnancy.
• Grade 3: Embryos with Grade 3 have moderate asymmetry or fragmentation. This means that the blastomeres may show more unevenness in size, and there may be a moderate amount of fragmentation present. Grade 3 embryos have a slightly lower chance of successful implantation and pregnancy compared to higher-graded embryos.
• Grade 4: Grade 4 embryos have significant asymmetry or fragmentation. The blastomeres may show significant variation in size, and there may be a substantial amount of fragmentation present. Grade 4 embryos have a lower chance of successful implantation and pregnancy compared to higher-graded embryos.

How important are embryo grades, anyway? 

Embryo grades provide valuable insights into the developmental potential of embryos, but they are not the sole determining factor of success. It’s important to recognize that grading systems primarily evaluate morphological characteristics and do not consider other crucial aspects, such as genetic abnormalities or the embryonic environment. 

While higher-graded embryos generally have higher implantation rates, it is not uncommon for lower-graded embryos to still result in successful pregnancies. The decision regarding embryo selection should involve a comprehensive evaluation of individual patient factors, including age, fertility history, and the number of available embryos.

How to know a "good" embryo grade

Understanding how to interpret embryo grades is vital for patients undergoing IVF. Typically, embryos with higher grades possess certain characteristics indicative of better developmental potential. For instance, in the Gardner system, high-quality blastocysts often have a fully expanded blastocoel, a tightly packed ICM with many cells, a well-formed TE with few or no fragments, and a smooth and even zona pellucida. These features suggest a greater likelihood of successful implantation and subsequent pregnancy. 

While these grading systems provide a standardized approach, there may still be some variability in interpretation between embryologists and clinics, based on that clinic’s own success rate data with embryos of different grades. Additionally, newer systems and modifications continue to emerge, aiming to improve the accuracy and reliability of embryo grading.

Understanding the grading system used by your fertility clinic and discussing it with your reproductive specialist can help you interpret and interpret the grading of your embryos more effectively. Remember, the grading system used may influence the way embryos are selected for transfer or cryopreservation.

Which embryo should I transfer first?

If you’ve made it to the point where you have more than one blastocyst embryo available to transfer, congratulations! That’s a huge weight off of your shoulders. Now, you’re likely focused on the next big decision: which embryo should I transfer first? Doctors may have different opinions on this, depending on various factors. Here are a few considerations as you and your doctor weigh this decision: 

Implantation potential

As we previously mentioned, higher graded embryos generally have a higher potential for successful implantation and pregnancy. They often exhibit better morphological characteristics and developmental quality, which can increase the chances of successful embryo implantation and subsequent pregnancy. Therefore, some clinicians may recommend transferring higher graded embryos first to maximize the likelihood of achieving a successful pregnancy.

Number of embryos

If you have a limited number of embryos available for transfer, some doctors may advise starting with the higher graded embryos. This approach aims to optimize the chances of achieving a pregnancy early in the treatment process, preserving any remaining higher graded embryos for future transfer attempts. 

Depending on how many kids you want, other doctors may advise the opposite — if you know you want two children, but you transfer a higher graded embryo earlier and get pregnant right away, that will delay your availability to potentially do another IVF cycle again should your lower-graded embryos not result in another pregnancy. Also, if you transfer a lower graded embryo and it does stick, that will only further increase your chances of a successful pregnancy down the line with another embryo, especially a higher graded one!

Patient-specific factors

Factors such as your age, fertility history, previous IVF outcomes, underlying medical conditions, and specific treatment goals should be taken into consideration. If you’ve undergone previous IVF cycles and had unsuccessful outcomes with higher graded embryos, your doctor might consider altering the approach and prioritize transferring lower graded embryos first to try something different. Although higher graded embryos generally indicate better chances of a successful pregnancy, it’s not impossible to have a baby using a lower graded embryo. They still show implantation potential and result in successful pregnancies in some cases.

We recommend talking with your REI to receive personalized guidance based on your specific situation. They can provide the most up-to-date clinical information, evaluate your individual factors, and offer recommendations tailored to your needs and timeline. 

Embryo grading for donor egg IVF cycles

With a donor egg IVF cycle, embryo grading is equally important. However, other factors, such as the intended parent or gestational carrier’s uterine environment and receptivity, still play a significant role in the success of donor egg IVF cycles. It’s also critical to understand that embryo grading is not indicative of the presence or absence of chromosomal abnormalities or specific genetic factors.

At Cofertility, the average number of mature eggs a family receives and fertilizes is 12. Some intended parents want to do two egg retrievals with the donor which is definitely possible. We also ask each of our donors whether they are open to a second cycle as part of the initial application — many report that they are!

You can see how many eggs are retrieved in the first cycle and go from there. If, for any reason, the eggs retrieved in that round do not lead to a live birth, our baby guarantee will kick in and we’ll re-match you at no additional match deposit or Cofertility coordination fee.

In conclusion

Embryo grading serves as a valuable tool in assessing the developmental potential and quality of embryos in assisted reproductive technology (ART). While higher-graded embryos generally exhibit higher implantation rates and better chances of success, embryo grade is not the end-all-be-all when it comes to predicting pregnancy outcomes. 

Grading systems primarily evaluate morphological characteristics and do not account for genetic abnormalities or the embryonic environment. Because of this, the decision regarding embryo selection should involve a comprehensive evaluation that considers multiple factors beyond just the assigned grade. Your fertility doctor can provide personalized guidance to optimize your chances of achieving a successful pregnancy based on their specific circumstances. We wish you all the best!