Monozygotic twinning: an eight-year experience at a large IVF center.

OBJECTIVE: To characterize incidence, chorionicity, amnionicity, and pregnancy outcome for monozygotic twin pregnancy (MZT) after IVF.

DESIGN: Retrospective review.

SETTING: University-based fertility center.

PATIENT(S): Autologous and oocyte donation IVF cycles eventuating in 4,976 clinical gestations from 2000 to 2007.

INTERVENTION(S): None.

MAIN OUTCOME MEASURE(S): MZT incidence, chorionicity, zygosity, pregnancy outcome.

RESULT(S): Ninety-eight MZTs were diagnosed after first-trimester ultrasound evaluation (2% incidence). The incidence in cycles transfering autologous oocytes was 1.7% but was 3.3% with donor oocytes; however, women <35 years old using their own oocytes displayed a similar rate (3.1%) to women using donor oocytes. 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 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). Ninety-five percent of all placental arrangements were confirmed as monochorionic-diamniotic on obstetric ultrasounds.

CONCLUSION(S): These findings confirm a higher incidence of MZT after IVF. Monochorionic-diamniotic implantations were increased, whereas monochorionic-monoamniotic were not. The MZT risk factors included young age and extended culture, but not zona penetration or cryopreservation.