Cleavage stage grading. What do we look for?
When grading blastocysts, we look for the size of the cavity, the inner cell mass, and the trophectoderm. However, when grading embryos on Day 2 and 3 and merely a collection of between 2 to 10 cells, we “grade” them by looking at the 3 main things:
# of Cells
On Day 2 we want to see 2-4 cells, preferably 4 cells. On Day 3 we want to see 6-10 cells, preferably 8 cells. If there are more or fewer cells than expected, the grade is reduced.
We do not want fragments. We see them, so we grade them. Some use a numerical score of 1 through 4, with 1 as good and 4 as bad. Others use a,b,c, or d. Either way, we prefer for no fragmentation.
We want the embryos cells to be the same size because as the embryo divides, the cells should split 50:50. Any deviation of this would bring down the grade and usually labs grade the same way as fragmentation i.e., 1-4 or a-d.
Based on these attributes, we rank the embryos against each other to determine how the culture is going. Some labs look at whether the embryo has started compacting on Day 3 also as this shows the genome activation as it heads towards blast. Note that the grading on Day 2-3 must not be relied on too heavily as the embryo will keep changing. It is becoming increasingly more obvious that Day 5-6 grading is more informative when deciding which embryo to transfer, and therefore why blast culture is generally suggested.