Effect of vanishing twin on singleton neonatal outcomes in frozen embryo transfer cycles

Objective: To investigate the association between vanishing twin syndrome (VTS) and adverse neonatal outcomes in singleton live births resulting from frozen embryo transfer (FET) cycles. Design: Retrospective cohort study. Subjects: This retrospective study involved a total of 14,583 women who underwent FET cycles that resulted in a live singleton newborn from January 2015 to June 2022. Exposure: Women were categorized into 2 groups: the VTS group (n = 1,078) and the non-VTS singleton group (n = 13,505). The women in the VTS group were propensity score matched with those in the non-VTS singleton group at a 1:3 ratio. Main Outcome Measures: The primary outcome was preterm birth (PTB) and low birth weight (LBW). Results: Our findings revealed that the incidence of PTB and LBW was significantly higher in the VTS group, with risk ratios of 1.53 (95% confidence interval, 1.23–1.91) and 1.76 (95% confidence interval, 1.34–2.32), respectively. Additionally, both gestational age and birth weight were significantly lower in the VTS group than in the non-VTS singleton group. Even when the analysis was restricted to participants with PTB and LBW, the VTS group still demonstrated lower gestational age and birth weight. Conversely, no statistically significant differences were observed in terms of cesarean delivery rate. Importantly, these results remained stable both before and after propensity score matching. Conclusion: Our study demonstrated that singleton live births in the VTS group are at increased risk of PTB and LBW compared with those in the non-VTS singleton group in FET cycles. Therefore, selective single embryo transfer should be fully considered, which can limit the problem originally.