Monozygotic Twins

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.

There is currently a European registry under the auspices of ESHRE that is recording all cases of MZT. Similar efforts in the US are underway. Until we get more data from the US centers and other centers around the world, it is important that patients are made aware of this potentially very serious complication of IVF/ICSI.