Background Monochorionic (MC) twins are at increased risk of fetal loss in early pregnancy as a consequence of twin-twin transfusion syndrome (TTTS). Previous studies estimated that the cumulative MC loss rate was >12% by 24 weeks' gestation in an era prior to routine fetoscopic laser treatment for TTTS. The aim of this study is to compare current fetal loss rates between MC and DC twins.

Methods A retrospective study of all twin pregnancy births of known chorionicity from a regional of 9 hospitals over ten year period. Ultrasound data was matched to hospital delivery records and a mandatory national register for perinatal losses (CMACE). Cumulative fetal and perinatal loss rates for MC and DC twins from 14 to 26 weeks gestation were analysed using Kaplan-Meier survival curves.

Results 3133 twin pregnancies (599 MC and 2534 DC) were included in the analysis. The total fetal loss rate in MC and DC twins were 6.3% (75/1198) and 0.9% (44/5068), respectively (OR 0.14, 95% CI 0.09 to 0.20). The cumulative rate of loss in MC twins rose significantly from 0.5% at 14 weeks' gestation up to a plateau of 6.3% at 24-26 weeks' gestation, whereas there was no significant change in DC pregnancy loss rate (0.5%) over this period.

Conclusion Cumulative fetal loss rate before 26 weeks' gestation in MC twins has almost halved compared to the rates available in the published literature. Early detection and prompt treatment of complications in MC twins is likely to have contributed to this improvement in twin pregnancy outcomes.