Embryo development begins a few hours after the formation of the zygote. The zygote divides in two cells that are called blastomeres. The next divisions follow, giving rise to more and smaller blastomeres. This process is called cleavage.
Two days after oocyte retrieval cleavage has started and embryos have undergone 2 cell divisions. Cleavage rates are high, about 95%. Typically, the first division occurs about 16 hours after fertilization (2-cell embryo). The second division happens 12 hours later (4-cell embryo) and divisions continue in an increasing rate. Between 4 and 8-cell stage the embryonic genome is activated. The next stages are the morula and the blastocyst.
Correct cleavage is important for embryo quality. The number and morphology of blastomeres, the presence of fragmentation, the morphology and fusion of pronuclei and the synchronous divisions are important features for the evaluation of embryos and are related to IVF outcome.
Therefore, embryos with late cleavage (e.g. 2-cell stage on day 2) are considered inferior to 4-cell stage embryos.