Congenital anomalies in twins: a register-based study.

BACKGROUND The risk of congenital anomalies in twins is higher than in singletons, but it is less well reported in relation to chorionicity. The aim of this study was to describe the prevalence of congenital anomalies in twin pregnancies by chorionicity and by major subtype and compare the rates with those in singletons. METHODS The study population included 2329 twin pregnancies (4658 twins) and 147,655 singletons delivered in the Northeast of England during 1998-2002. Data were obtained from the population-based Northern Multiple Pregnancy Register and Northern Congenital Abnormality Survey. RESULTS The rate of congenital anomalies in twins was 405.8 per 10,000 twins versus 238.2 per 10,000 singletons [rate ratios (RR) = 1.7, 95% confidence interval (CI) 1.5-2.0]. In twins with known chorionicity (84.8% of all twins), the prevalence of congenital anomalies in monochorionic (MC) twins (633.6 per 10,000) was nearly twice that in dichorionic (343.7 per 10,000; RR = 1.8, 95% CI 1.3-2.5). There was an increased rate of congenital anomalies in twin compared with singleton pregnancies for all major types of anomalies, except chromosomal abnormalities. CONCLUSIONS This study using high quality, population-based data on multiple pregnancies and congenital anomalies found that twins, particularly MC twins, have a higher risk of congenital anomalies than singletons.