There are several embryo grading systems used by embryologist. These systems evaluate the embryos before transfer. The American Society of Reproductive Medicine (ASRM) has established a standard grading system for embryos, which is often adapted by the fertility center. While not universally used, a standardized grading system allows for evaluation and scoring of any stage of the embryo development, which extends from initial fertilization to formation of a blastocyst.
The Grading Process
After the eggs have been retrieved, they are placed in an incubator for several hours. The embryologist then checks the eggs for maturity by removing 1-2 cells from each egg (called cumulus cells). Each mature egg can undergo intracytoplasmic sperm injection (ICSI) or be combined with sperm in a petri dish for fertilization. The following day, an embryos form, which are checked for fertilization. Depending on which day embryo transfer is planned (day 3, 4, or 5), the embryologist grades the embryos and chooses which are best for transfer and/or cryopreservation (freezing).
Day 3 Embryo Grading
Day-3 embryos are graded based on the number of cells that make up the embryo, the symmetry of each cell, the amount of fragmentation, and other factors. A quality day-3 embryo will have eight cells, no fragmentation, and symmetrical cells. This is considered an 8AG embryo.
Cells – An embryo should have between 7-9 cells on day 3, with the ideal embryo being at least 8 cells. However, embryos with 7 or 9 cells can result in a successful pregnancy. Embryos with 5 cells or less have lower success rates.
Symmetry – This is the medical term for size and contour of the cells. An embryo cells should all be the same size. Embryos with symmetrical cells are rated as “good,” whereas ones with cells of different sizes are considered “fair.” “Poor” embryos are those with cells of varying sizes.
Fragmentation – This is when parts of a cell in the embryo break apart and appear as small blebs (fragments) within the embryo. The degree of fragmentation is related to the overall quality of the embryo. If an embryo has no fragmentation, it receives an “A.” An embryo with a greater percentage of fragmentation may receive a score of “C” or “D.” If the embryo has a score of D, it is not used for cryopreservation or transfer.
Other grading factors – Certain things contribute to embryo grading, but these factors may not be part of the scoring system. Other grading factors include appearance of the zona pellucida (the outer shell may be too hard), cytoplasm is granular (the inner gel material of the cells are grainy), and presence of vacuoles.
Blastocysts are day-5 embryos. Scoring of blastocysts is similar to scoring of day-3 embryos. However, they have a completely different set of scoring criteria. Grading evaluates:
Quality of the blastocyst – This is determined by assessing the outer ring of cells, which is called the trophectoderm. These cells eventually form into the placenta.
ICM quality – The embryologist checks the inner cell mass (ICM), which is composed of stem cells that form into the fetus.
Degree of expansion of the blastocyst cavity – If or not it has properly expanded.
Hatching – If or not the blastocyst has started to break out of the zona pellucida (shell around the embryo).