Vanishing twins: a predictor of small-for-gestational age in IVF singletons.

BACKGROUND The purpose of this study was to assess the effect of a vanishing twin on the risk of being small-for-gestational age (SGA) in in vitro fertilization (IVF) singletons. METHODS The study included 642 survivors of a vanished co-twin, 5237 primary singletons and 3678 primary twins. The survivor cohort was subdivided according to gestational age at the time of vanishing to give groups of early (<8 weeks), intermediate (8-22 weeks) and late (>22 weeks) survivors. RESULTS The rate of SGA infants was significantly higher in survivors than in singletons (OR: 1.50, 95%CI: 1.03-2.20) and a significant inverse correlation was observed between SGA and the gestational age at the time of vanishing (r = -0.10, P < 0.02). Also in term infants, the risk of birthweight <2500 g was higher in survivors than in singletons (OR: 1.71, 95%CI: 1.06-2.74). A similar increase in the rate of low birthweight in term survivors was seen with increasing gestational age at the time of vanishing (r = -0.12; P < 0.01). In multiple logistic regression analysis adjusting for maternal age, parity, child gender and pre-eclampsia, the vanishing of a co-twin (OR: 1.56, 95%CI: 1.06-2.27) and gestational age at the time of vanishing (OR: 2.08, 95%CI: 1.00-4.35) were the only significant predictors of being SGA. CONCLUSIONS IVF singletons with a vanished co-twin had a higher rate of SGA than singletons from a single gestation and the risk of SGA is increased with increasing gestational age at the time of vanishing.