What makes them split? Identifying risk factors that lead to monozygotic twins after in vitro fertilization.

To identify the incidence, risk factors, and obstetric/perinatal outcomes associated with monozygotic twins (MZTs) after IVF. Nested case-control. University-based center. The IVF cycles eventuating in pregnancy from 2000-2009. None. The MZT incidence, chorionicity/zygosity, pregnancy/neonatal outcome. Of 6,223 gestations, 131 MZTs were diagnosed (2.1% incidence; 2.0% in autologous and 2.7% in donor IVF cycles), 10 were dichorionic, and 121 were monochorionic. Controlling for all risk factors, young oocyte age, extended culture (noncleavage embryos transferred on/after day 4), and year of IVF treatment cycle were significantly associated with MZT. When assessing factors associated with specific MZT placentation, day 3 assisted hatching correlated more with dichorionic MZT, whereas extended culture and advanced day 5 embryonic stage correlated with monochorionic MZT. Comparing monozygotic to dizygotic multigestation outcomes, MZT fared worse; however, once controlling for triplet gestation, only gestational age at delivery remained significantly compromised in the monozygotic group. After IVF the incidence of MZT is high, with young oocyte age, year of treatment, and extended culture (or embryo stage at transfer) conferring greatest risk. Regarding MZT type, assisted reproductive technology (ART) procedures may influence the timing of embryonic splitting (i.e., division in early embryonic development may be influenced by zona pellucida [ZP] manipulation whereas later splitting may occur during delayed implantation). Poor obstetric/perinatal outcome is significantly impacted by the presence of an "extra" fetus, as high-order multiple gestation concurrent with an MZT conveyed the worst prognosis.