We explored the relationship of umbilical cord insertion and fusion of placentas with birthweight in monozygotic monochorionic (MZ MC), monozygotic dichorionic (MZ DC), and dizygotic (DZ) twins. In addition, we evaluated some of the possible factors responsible for the restricted intrauterine growth of twins compared with singletons. The birthweight of 4529 liveborn twin pairs of the East Flanders Prospective Twin Survey was prospectively recorded, placentas were examined, and site of umbilical cord insertion was determined after delivery. Birthweight of 76 490 liveborn singletons was obtained from the Study Centre for Perinatal Epidemiology (SPE). Infants with a peripheral cord insertion weighed 150 g less (P < 0.001) than infants with a central cord insertion. DZ infants had a significantly (P < 0.001) higher incidence of central cord insertion than MZ DC and MZ MC infants. MZ DC infants with fused placentas and a peripheral cord insertion weighed on average 300 g less (P < 0.01) than infants with separate placentas and a central cord insertion. In DZ infants, fusion of the placentas did not affect birthweight. Twins gain less weight per week of gestation than singletons from 32 weeks onwards (twins: 128 g, 156 g, 75 g and singletons: 118 g, 251 g, 149 g, weeks 27-31,32-36, 37-42 respectively). From week 32 onwards, parity, birth rank, cord insertion and number of placentas also influenced birthweight of twins. We conclude that the difference between the birthweights of DZ, MZ DC, and MZ MC infants may originate from the least favourable antenatal situation, namely fused placentas with a peripheral cord insertion, which occurs most frequently in MZ twins. Gestation is the main determinant of birthweight. Other placental and maternal factors have a modest but significant influence on prenatal growth.