It does, at least according to a study in the July 2010 issue of Fertility & Sterility.
The authors surveyed 4976 clinical pregnancies from one large IVF center and found ninety-eight cases of monozygotic twins (MZTs) were diagnosed after first-trimester ultrasound evaluation (2% incidence). In naturally conceived pregnancies the incidence is about 0.25-0.5%.
The incidence in women transfering embryos from their own eggs was 1.7% but was 3.3% with donor eggs; however, younger women (<35 years old) using their own eggs displayed a similar rate (3.1%) to women using donor eggs.
Eighty MZTs occurred after fresh day-5 transfer; only 14 followed fresh day-3 transfer (2.6% vs. 1.2%). The MZT incidence in day-3 transfers without assisted hatching was not different from those with hatching (1.3% vs. 1.1%). In addition, MZT incidence did not differ significantly whether or not ICSI was performed (2.4% vs. 2.0%). Four MZTs occurred after frozen-thawed embryo transfer (0.8% incidence).
The good news is that ninety-five percent of all placental arrangements were confirmed as having two amniotic sacs on obstetric ultrasounds, making them safer pregnancies than when both babies are in the same amniotic sac.
The take-home message is that this is a good reason for young women and those using eggs from a young donor to seriously consider elective single embryo transfer (eSET) so if a set of identical twins does occur and the other embryo implants one does not have to deal with a triplet pregnancy.