Maternal age is associated with embryo splitting after single embryo transfer: a retrospective cohort study.

PurposeTo determine whether maternal age has an impact on monozygotic twinning (MZT) rates in women undergoing single embryo transfer (SET).MethodsThis is a retrospective cohort study analyzed for the incidence of MZT of all clinical pregnancies after a single embryo transfer was carried out between 2014 and 2018. The effect of different assisted reproductive technology (ART) parameters on the incidence of MZT was evaluated.ResultsThere were a total of 8459 cycles resulting in pregnancy during the study period. Of these pregnancies, 8236 were singletons and 223 were MZT. The preterm birth rate, miscarriage rate, and cesarean section rate were higher in MZT. Birth weight and gestational age at delivery were lower and smaller. In the univariate analysis, the risk of MZT was decreased with frozen embryo transfer (ET). A nonlinear relationship was observed between maternal age and MZT. A negative relationship between maternal age and MZT was observed in the patients' age ≥ 36 years.ConclusionAdvanced maternal age was associated with a lower rate of MZT. A threshold female age of 36 years existed for lower MZT.