Consequences of vanishing twins in IVF/ICSI pregnancies.

BACKGROUND Spontaneous reductions are a possible cause of the increased morbidity in IVF singletons. The aim of this study was to assess incidence rates of spontaneous reductions in IVF/ICSI twin pregnancies and to compare short- and long-term morbidity in survivors of a vanishing co-twin with singletons and born twins. METHODS We identified 642 survivors of a vanishing co-twin, 5237 singletons from single gestations and 3678 twins from twin gestations. All children originated from pregnancies detected by transvaginal sonography in gestational week 8. By cross-linkage with the national registries the main endpoints were prematurity, birth weight, neurological sequelae and mortality. RESULTS Of all IVF singletons born, 10.4% originated from a twin gestation in early pregnancy. Multiple logistic regression analyses adjusted for maternal age, parity and ICSI treatment showed for birth weight <2500 g an odds ratio (OR) of 1.7 [95% confidence interval (CI) 1.2-2.2] and for birth weight <1500 g OR 2.1 (95% CI 1.3-3.6) in singleton survivors of a vanishing twin versus singletons from single gestations; corresponding figures were seen for preterm birth. This increased risk was almost entirely due to reductions that occurred at >8 weeks gestation. We found no excess risk of neurological sequelae in survivors of a vanishing co-twin versus the singleton cohort; however, OR of cerebral palsy was 1.9 (95% CI 0.7-5.2). Furthermore, we observed a correlation between onset of spontaneous reduction, i.e. the later in pregnancy the higher the risk of neurological sequelae (r = -0.09; P = 0.02). Adjusted OR of child death within the follow-up period was 3.6 (95% CI 1.7-7.6) in the survivor versus the singleton cohort. CONCLUSIONS One in 10 IVF singletons originates from a twin gestation. Spontaneous reductions that occur at >8 weeks gestation are one of the causes for the higher risk of adverse obstetric outcome in IVF singletons.