As embryologists, one of the most common questions that we get from patients is “What do the grades of my embryos tell us about my chances of becoming pregnant?” The answer to this question is not a simple one. The objective of this article is to explain how we grade embryos and what those grades mean as far as an embryo’s potential for development.
All embryo grading systems are subjective. While we can make educated guesses about an embryo’s potential based on the experience of many embryologists grading millions of embryos, there are many cases of embryos with poor grades that make pregnancies and perfect embryos that do not. Also, no matter the grading system, the embryo grades do not tell us what is going on inside the embryo genetically.
We use grading systems to help us determine which embryos to transfer and/or freeze. At the Texas Fertility Center, embryo transfers occur either 3 days or 5 days after a retrieval. Because embryos are developmentally different on these days, we have different grading systems for day 3 (cleavage stage) embryos and day 5 (blastocyst stage) embryos. Each will be discussed in turn.
Day 3 Embryo Grading:
Day 3 embryos are referred to as “cleavage stage” embryos. The reason for this designation is that the cells in the embryo are dividing (or cleaving) but the embryo itself is not growing in size. Think of a pizza. When you slice it, you create more pieces of pizza, but you do not increase the size of the pizza itself. This is what a cleavage stage embryo is like. The genetic material replicates and the cells divide, but the volume of the embryo does not differ from the volume of the unfertilized egg.
Conceptually, you would think that embryos would divide in a very specific sequence; one cell becomes two; two cells become four; four cells become eight and so on. However, actual embryos do not divide synchronously. We commonly see three, five, six cell, etc. embryos. This is not an indication of a poor embryo, but of one that is growing normally. Also, as embryos divide, sometimes a small portion of cytoplasm (the inside of the cell) breaks off and forms a bleb that we call a fragment. Fragments do not contain nuclei and are not considered cells. The causes of fragmentation are poorly understood, but embryos containing a lot of fragmentation are developmentally disadvantaged simply because the cells lose too much cytoplasm and thus, cellular machinery, to the fragments.
Cleavage stage embryos are graded using 2 criteria: the number of cells in the embryo and their appearance under a high-power microscope. While the cell number is objective, the score for appearance is subjective using a score of 1-4. Typically, a good, normally growing day 3 embryos will contain between 6 and 10 cells. From studies that we have done in our lab and from other published studies, we know that embryos that contain these numbers of cells are more likely to develop into viable blastocysts than embryos with fewer cells.
The embryo grade refers to how the cells in the embryos look. A grade one embryo, for example, is one in which all of the cells are the same size and there is no fragmentation in the embryo. The system we use to grade embryo appearance is presented in the following table.
|Grade 1||Cells are of equal size; no fragmentation seen|
|Grade 2||Cells are of equal size; minor fragmentation only|
|Grade 2.5||Cells are mostly of equal size; moderate fragmentation|
|Grade 3||Cells are of unequal size; no fragmentation to moderate fragmentation|
|Grade 4||Cells are of equal or unequal size; fragmentation is moderate to heavy|
Grade 1 through 2.5 embryos seem to have the greatest potential for developing to the blastocyst stage. However, a grade 3 embryo may also be of good quality if its appearance can be explained by asynchronous cell division rather than by poor development. We have published data showing that the number of cells in the day 3 embryo is a better indicator of potential than the grade of the embryo. Therefore, an 8 cell Grade 3 embryo would have better potential than a 4 cell grade 2 embryo on day 3.
Day 5 Embryo Grading:
On day 5, embryos are continuing to divide and the number of cells continues to increase, but the cells are also growing and differentiating into specific cell types. By this time embryos should have started to outgrow the space inside the zona pellucida (ZP or “shell”) that surrounds the embryo. They start to expand and thin the ZP in preparation for the stage of development when the blastocyst bursts through this membrane (“hatching”) to prepare for implantation into the uterine lining.
There are two cell types in the day 5 embryo or blastocyst. One cell type forms the Inner Cell Mass (ICM). This ball of cells will eventually grow into the fetus. The other cell type is the Trophectoderm Epithelium (TE). This sheet of cells will go on to make tissues needed during pregnancy (like the placenta). Together these cell types make a fluid filled sphere with the TE cells on the outside and the ICM inside. Think of a balloon. If you blow up a balloon and put a ping-pong ball inside, that is what a blastocyst looks like. The latex of the balloon is the TE and the ping-pong ball is the ICM. Both of these cell types are necessary to establish a healthy pregnancy. You cannot have a baby without a placenta and you cannot have a pregnancy without a fetus, so when we grade embryos at the blastocyst stage, we assign a letter grade to each of the cell types as well as to the fluid filled cavity or blastocoel. We also assign a grade to designate how much the embryo is expanded (this relates to how large the blastocoel is as well as how many cells are contained in the embryo). Examples of the expansion grades are:
- Very Early Blastocyst, in which the cavity is just beginning to form in the embryo and the cell types are not yet distinguishable;
- Expanded Blastocyst, in which the cavity is fully formed, the embryo contains 100 to 125 cells, but is still contained within the thinned ZP, and
- Hatched Blastocyst, in which the embryo is outside of the ZP, and contains upwards of 150 cells.
Below are some examples of embryos that we typically see on day 5.
50-75 cells, Developing ICM, growing (DCB) trophectoderm,
100-125 cells, compacted, good quality (CBB) ICM,
100-125 cells with trophectoderm beginning (BBB)
100-125 cells with trophectoderm beginning Hatching Blastocyst
As you might notice from the examples above, blastocyst grading is complex and therefore there are no absolute grades. While an A is “better” than a D, an embryo with a D grade ICM, for example, may be still developing and when viewed later, the ICM may have compacted into a B or even an A. Also, expansion grades are indicators of growing embryos. Many times a Very Early Blastocyst on Day 5 becomes an Expanded Blastocyst on Day 6 and may be frozen if the other indicators are also good. The determination of whether an embryo has good potential or not is made by taking all of the components of the embryo into account.
Embryo grading is a tool. It is a tool that the physicians and embryologists use along with a patient’s age, fertility history, and other information to determine the optimal day of transfer, the appropriate number of embryos to transfer, and exactly which embryos to transfer.Contact ARC to Talk to a Fertility Specialist near You