Understanding the Embryo Grading Process in IVF

When you’re starting out on your IVF journey, understanding the many different steps of the experience can feel overwhelming. Embryo grading is one step that can leave you with more questions than answers. What does a high or low-grade embryo really mean? And how does this impact your chances of getting pregnant? Let’s walk through what each element of embryo grading means to help you feel more prepared.

What is Embryo Grading?

Embryo grading is a method Reproductive Endocrinologists use to assess the quality of embryos before transferring them to the uterus or freezing your embryos for future use. The goal of grading is to predict which embryos have the best chance of leading to a healthy pregnancy. 

It’s important to remember that while grading provides helpful information, it’s not the sole indicator of your chances of having a healthy baby. Even embryos with lower grades can lead to healthy pregnancies, so this process should be viewed as part of a larger picture.

How is Embryo Grading Done?

Embryo grading is usually performed on two different days after your eggs have been fertilized: Day 3 and Day 5 (or sometimes Day 6). Here's what happens on these key days:

• Day 3: On the third day after fertilization, the embryo typically has about 6-8 cells. At this stage, embryologists evaluate the number of cells the embryo has divided into, the symmetry of these cells, and the level of fragmentation (small pieces of cytoplasm that are breaking off from the embryo). An ideal Day 3 embryo would have 8 evenly sized cells and little to no fragmentation.

• Day 5/6: This is the stage when the embryo reaches the blastocyst stage. By now, it should have developed two distinct cell types—one type that will become the fetus, and the other type that will form the placenta. The grading at this stage is based on three factors:

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

2. The trophectoderm (TE), which develops into the placenta.

3. The degree of expansion of the embryo, indicating how much it has grown and filled with fluid.

Embryo Grading Scale

Doctors use different scales to grade embryos, but a common system for blastocysts includes a combination of letters and numbers. For example, an embryo might be graded as 4AA. Here's what that means:

• The number (1 to 6) refers to the degree of expansion of the blastocyst. A 1 means the blastocyst is still compacting, and a 6 means it has fully hatched out of its shell, which is preferable.

• The first letter (A, B, or C) represents the quality of the ICM. An A-grade inner cell mass, the highest level of grading, means that the ICM has many cells packed closely together. 

• The second letter (A, B, or C) refers to the quality of the TE. Similarly, A is the highest rating, and it means that the trophectoderm has many tightly packed cells, while C a grade means that the TE has fewer, more irregular cells.

For example, a 4AA embryo is nearly perfect, with an expanded blastocyst and top-quality cells both in the ICM and the TE.

Does Embryo Grade Affect IVF Success?

While embryo grading is an important tool for doctors, it’s not a definitive predictor of success. It’s tempting to get caught up in the numbers and letters, but many people have had successful pregnancies with lower-graded embryos.

High-grade embryos are associated with a higher likelihood of implantation and pregnancy, but lower-grade embryos can still lead to healthy babies. For this reason, Reproductive Endocrinologists use grading alongside other factors, such as your age, medical history, and overall fertility health, to make the best decision for you.

Fresh vs. Frozen Embryo Transfers

Another aspect to consider is whether you will be undergoing a fresh or frozen embryo transfer. In fresh cycles, embryos are graded and transferred within days of fertilization, whereas in frozen cycles, the best embryos are selected, frozen, and then transferred at a later time. In the case of frozen embryo transfers, this means your Reproductive Endocrinologist can grade your embryo at a slightly later stage of development, which might give a more accurate assessment of your embryo’s likelihood of successfully implanting.

Freezing doesn’t negatively impact embryo quality—thanks to advanced technology like vitrification (a fast-freezing method)—and some studies suggest frozen embryo transfers can result in higher pregnancy rates compared to fresh transfers.

What Does This Mean for Me?

It’s common to want to focus on the grade of your embryos, but it’s important to remember that embryo grading is just one single step of your fertility journey. A lower-grade embryo doesn’t mean failure, just as a high-grade embryo doesn’t guarantee success. Trust your doctor to use their expertise and experience to make the best recommendations for your individual case.