Vanishing twin syndrome and fetal reduction adversely affect perinatal outcomes following IVF-FET: an analysis based on 33,238 ongoing pregnancies.

purposeTo assess the associations between vanishing twin (VT) and fetal reduction (FR) with obstetric and perinatal outcomes following in vitro fertilization (IVF)-frozen embryo transfer (FET).

methodsThis was a retrospective cohort study involving women who had undergone FR or experienced VT during the period from 2012 to 2022. Cohorts were stratified by timing into early (< 15 weeks) and late (≥ 15 weeks) VT/FR. Controls comprised primary singletons and non-reduced twins. The primary outcome measurements were maternal and birth complications.

resultsAmong 33,238 ongoing pregnancies, there were 24,316 primary singletons, 7452 non-reduced twins, 1354 VTs, and 116 FRs. Multivariable analyses showed birth outcomes in the study groups were similar to or better than non-reduced twin deliveries. Compared to primary singletons, both early and late FRs were associated with increased risk of preterm birth (PTB); late FR also increased the risk of low birthweight (LBW). Early and late VTs similarly had higher risks of PTB and LBW versus primary singletons. Obstetric complications were generally comparable or lower in the study groups versus twin deliveries; however, late FR was linked to a higher risk of hypertensive disorders of pregnancy compared with primary singletons, and late VT was associated with increased abnormal placentation versus primary singletons and twins.

conclusionsIn this large IVF-FET cohort, most birth and maternal outcomes were comparable or better than in non-reduced twins, but certain complications remained more common in both VT and FR groups. Both exposures were linked to adverse perinatal outcomes versus primary singletons. Moreover, VT and FR appear to be more problematic when these occur later in pregnancy.