Background Twinning rates are rising due to a wider use of assisted conception and increasing maternal age. The higher risk of perinatal mortality in twins compared to singletons remains of concern.

Aim To compare cause-specific stillbirth and neonatal mortality in dichorionic and monochorionic twins.

Methods The study used data on twins delivered in the North of England during 1998–2007 and notified to the population-based Northern Survey of Twin and Multiple Pregnancy (NorSTAMP). Ascertainment is from the earliest diagnosis of a multiple pregnancy. NorSTAMP records are linked to regional Perinatal Mortality Survey.

Results There were 4565 twin maternities with an average twinning rate of 14.9 per 1000 maternities. The overall stillbirth and neonatal mortalities in twins over the 10 years were 18.0/1000 births and 23.0/1000 live births, respectively, which were 3.5 and 9 times those in singletons. Both stillbirth and neonatal mortalities were significantly higher in monochorionic than in dichorionic twins: 44.4 vs 12.2 per 1000 births, rate ratio (RR) 3.6, 95% CI 2.7 to 5.0 and 32.4 vs 21.4 per 1000 live births, RR 1.5, 95% CI 1.1 to 2.1, respectively. The much higher stillbirth rate in monochorionic twins was due to predominance of antepartum deaths resulting from antepartum hypoxia, twin-twin transfusion and congenital anomalies. There was no improvement over time in stillbirth rates in either dichorionic or monochorionic twins, while neonatal mortality significantly declined in dichorionic twins.

Conclusions Both dichorionic and monochorionic twins remain of higher risk for stillbirth and neonatal mortality than singletons, with monochorionicity contributing to much higher risk.