There has been an increase in the occurrence of monozygotic twinning secondary to the use of assisted hatching, ICSI, but mostly, the transfer of blastocysts on day 5-6 during IVF. Monozygotic twinning (MZT) occurs when an embryo splits after fertilization, resulting in identical twins. The incidence of MZT is 0.42% naturally (in normal pregnancies). With the use of assisted reproduction, the incidence of MZT has increased, and currently varies between 1.2% and 8.9%. A recent report suggests that the incidence of MZT is higher when blastocysts resulting from ICSI cycles are replaced compared to blastocysts resulting from IVF (Tarlatzis et al, Fertil Steril 77:196-8, 2002).
Splitting of the embryo may occur at any time during the first 2 weeks after fertilization, resulting in several forms of MZT. In about one third of the MZT cases, the splitting occurs within 72 hours of fertilization and the placenta will be dichorionic-diamniotic (i.e two placentae and two amniotic sacs). In about 2/3 of the MZT cases, the splitting occurs 4-8 days after fertilization, and the placenta will be monochorionic-diamniotic (i.e one placenta and two amniotic sacs). In 5% of the MZT cases, the splitting occurs 8-13 days after fertilization, resulting in monochorionic-monoamniotic twins (one placenta and one amniotic sac). Rarely, the splitting occurs after 13 days, resulting in conjoined twins.
We have recently sumbitted our experience over the past 5 yrs with MZT for publication, and our paper will appear in the prestigious Fertility and Sterility Journal in 2009. We only had 4 cases of MZT in 374 Blastocyst transfer cases (1.57% incidence) as compared to no cases in those women receiving a day 3 ET (0/122). The pregnancy rates were significantly higher in those who received a blastocyst transfer (67.8%) (those are the best prognosis patients) as compared to 38% in those having a day 3 transfer. Such a low incidence of MZT should encourage good prognosis patients to proceed with Blastocyst transfers to maximize their pregnancy rates and decrease the number of transferred embryos. To view our article please click on the link below:
/media/assets/MZT paper-corrected proofs.pdf