Embryo transfer can be performed after various durations of embryo culture, conferring different stages in embryogenesis. The main stages at which embryo transfer is performed are cleavage stage (day 2 to 4 after co-incubation) or the blastocyst stage (day 5 or 6 after co-incubation).
Because in vivo, a cleavage stage embryo still resides in the fallopian tube and it is known that the nutritional environment of the uterus is different from that of the tube, it is postulated that this may cause stress on the embryo if transferred on day 3 resulting in reduced implantation potential. A blastocyst stage embryo does not have this problem as it is best suited for the uterine environment  [https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD002118.pub5/full]
Embryos who reach the day 3 cell stage can be tested for chromosomal or specific genetic defects prior to possible transfer by preimplantation genetic diagnosis (PGD). Transferring at the blastocyst stage confers a significant increase in live birth rate per transfer, but also confers a decreased number of embryos available for transfer and embryo cryopreservation, so the cumulative clinical pregnancy rates are increased with cleavage stage transfer. It is uncertain whether there is any difference in live birth rate between transfer on day two or day three after fertilization.
Monozygotic twinning is not increased after blastocyst transfer compared with cleavage-stage embryo transfer.
There is a significantly higher odds of preterm birth (odds ratio 1.3) and congenital anomalies (odds ratio 1.3) among births having reached the blastocyst stage compared with cleavage stage. Because of increased female embryo mortality due to epigenetic modifications induced by extended culture, blastocyst transfer leads to more male births (56.1% male) versus 2 or 3 day transfer (a normal sex ratio of 51.5% male).