One of the discussions we may have with patients prior to a blastocyst transfer is the supposed increased chance of an embryo at the blastocyst stage splitting and becoming an identical twin pregnancy.
In this study in the February 2010 issue of Fertility and Sterility, the authors monitored almost 2000 IVF cycles and found in this group of single-embryo transfers there was no statisically significant difference observed in the probability of monozygotic twinning between the cleavage-stage (Day 3) and the blastocyst group (Day 5). The odds of an identical twin pregnancy was 2.6% in the cleavage-stage group (8/308) and 1.8% (5/271) in the blastocyst group The odds ratio for the incidence of monozygotic twinning after Day-5 embryo transfer was calculated to be 0.71 compared to the Day 3.
Despite the stage of the embryos at the time of transfer, one must be mindful of the almost 2% chance of monozygotic twins when selecting the number of embryos to transfer.