Blastomere

After an egg is fertilized, it divides into a ball of cells called blastomeres. On day 3 a normal embryo has about 8 blastomeres. Embryologists grade these cells for number, size uniformity, and fragmentation — cytoplasmic fragments that appear between cells.

High fragmentation (greater than 25% of embryo volume) is associated with lower implantation rates. Day-3 embryo transfers use blastomere grading to select the best embryos; however, most programs now extend culture to the blastocyst stage for better selection. Blastomere biopsy for PGT-A was the original PGT technique, now largely replaced by trophectoderm biopsy at the blastocyst stage.