Background: Chorionicity is one of the main predictors of higher perinatal mortality in twins. The aim of this large population-based study was to analyse stillbirth and neonatal mortality by cause of death and chorionicity and to quantify the risk of stillbirth by gestational age in dichorionic (DC) and monochorionic (MC) twins.
Methods: We used data on twin maternities delivered in the North of England from 1998 to 2007 and notified to the Northern Survey of Twin and Multiple Pregnancy. Prospective risk of stillbirth by gestational age at death was calculated using number of stillborn fetuses at or beyond a given gestational period per 1000 fetuses in ongoing pregnancies.
Results: There were 4565 twin maternities (9130 twins) with an overall twinning rate of 14.9 per 1000 maternities. The overall stillbirth and neonatal mortality rates in twins during 1998-2007 were 18.0/1000 births and 23.0/1000 live births, respectively. Stillbirth and neonatal mortality rates were significantly higher in MC than DC twins: 44.4 versus 12.2 per 1000 births [relative risk (RR): 3.6; 95% CI: 2.6-5.1], and 32.4 versus 21.4 per 1000 live births (RR: 1.5; 95% CI: 1.04-2.2), respectively. There was no significant improvement over time in either stillbirth or neonatal mortality rates in either group. The prospective risk of antepartum stillbirth was higher for MC than DC twins at all preterm gestations and the highest risk was before 28 weeks' gestation.
Conclusions: MC twins have higher rates of stillbirth and neonatal mortality than DC twins, and rates did not improve over 1998-2007. The prospective risk of antepartum stillbirth is much higher for MC twins at all gestational ages.