Objective To assess the relation of umbilical cord insertion and fusion of placentas with birthweight in monozygotic monochorionic, monozygotic dichorionic, and dizygotic twins.

Design A prospective population study.

Population 4529 liveborn twin pairs whose birthweight was recorded, placentas were examined, and site of umbilical cord insertion was determined after delivery.

Results Infants with a peripheral cord insertion weighed 150g less (P<0.001) than infants with a central cord insertion. Dizygotic infants had a significantly (P<0.001) higher incidence of central cord insertion (82%) than monozygotic dichorionic (65%) and monozygotic monochorionic (53%) infants. Monozygotic dichorionic infants with fused placentas and a peripheral cord insertion weighed on average 300g less (P<0.01) than infants with separate placentas and a central cord insertion. In dizygotic infants, fusion of the placentas did not affect birthweight.

Conclusions Umbilical cord insertion is associated with birthweight in all types of twins. Fusion of the placentas only affects the birthweight of monozygotic dichorionic, not that of dizygotic infants. This might be due to the greater proximity of implantation of monozygotic dichorionic twins. Dizygotic twins weighed more than monozygotic twins only when placentas were fused and cord insertion was peripheral. Hence, the difference between the mean birthweights of dizygotic, monozygotic dichorionic, and monozygotic monochorionic infants may originate from the least favourable antenatal situation, namely fused placentas with a peripheral cord insertion, which occurs most frequently in monozygotic twins.