Understanding Embryo Grading for Egg Donors
Once your egg retrieval is complete, it is quite possible that you walk out the clinic and never know what becomes of the eggs you donated. For other egg donors, however, the burning desire to understand what happens next leads to questions about the remainder of the eggs’ development.
Once the embryos are created, doctors will watch them very closely to see how they divide and develop. The designation of growth that is given is often called an embryo grade.
Embryo grading is assigned based on several factors, the first being the day in which the grade is given. Grades are typically assigned on either day three or day five of growth. Given the fact that the amount the embryo has grown differs drastically from day three to day five, the method for grading will differ as well.
Day 3 Grading
During this phase of development the cells are divide rapidly. You should note that a dividing cell in this phase of development is not growing. All cells at this stage will be roughly the same size, but the number of divisions within the cell may vary greatly.
As the cells divide, it is worth noting that occasionally some of the cytoplasm, the inside of the cell, will break off and form smaller “fragments” that do not contain a nuclei and are not considered a true “cell.” These fragments are not abnormal and do not immediately suggest poor embryo quality.
Day 3 cells will be graded based on the number of cell divisions visible compared to the number of fragments that are visible. Typically doctors hope to see between six and ten equally sized cells and an absence of fragments in an ideal day 3 embryo.
The doctor will continue to look at cell divisions and fragmentations and assign a numerical grade between 1 and 4. Doctors will expect embryos with a grade of 1- 2.5 to have the most likely chance of continuing to develop into a day 5 blastocyst.
Day 5 Grading
Most doctors will wait to issue a grade until the embryo reaches the blastocyst stage on day 5 of development. While cells continue to divide, they now begin growing in volume. This means that they may outgrow their “shell” and begin to hatch in preparation for implantation into the uterine lining.
Another development on day five is the new presence of two kinds of cells. The inner cell mass (ICM) which will eventually turn into the fetus, and the Trophectoderm Epithelium (TE) which will eventually form necessary tissues needed for pregnancy, including the placenta. Doctors look for both types of cells and judge their appearance when issuing a day five embryo grade. Other issues to note include the fluid cavity between the two cell types and how much the embryo has expanded.
Once they’ve finished evaluating the embryo, doctors will issue a letter grade to the blastocyst. Most doctors will issue a grade for each type of cell, resulting in a two-letter grade. Other doctors will also give a third letter grade to correspond with the growth. As a result, the best quality day five embryo will receive a grade of either “AAA” or “AA.”
Doctors will then consider the grade of each component of the cell when deciding whether they believe the embryo is likely to continue growing and result in a positive pregnancy. The grading system at this stage can be quite subjective and is therefore not a guarantee. Even the best-graded embryos sometimes fail to implant and grow while, embryos with a lower grade may develop into a healthy pregnancy. It is important to remember that grading is just a tool, not a promise of success.