Background Research consistently suggests that compared to singletons, twins are at increased risk of congenital anomalies. The risk of congenital anomalies amongst twins that share a placenta, monochorionic (MC) twins, reportedly exceeds that of twins that do not share a placenta, dichorionic (DC) twins. However, the risk of congenital heart disease (CHD) amongst twins is less well researched, particularly in relation to chorionicity. The aim of this study was to calculate the risk of CHD in twins and to examine the relative risk (RR) of CHD in twins compared to singletons, according to chorionicity and CHD severity.

Methods Twins and singletons with CHD notified to the Northern Congenital Abnormality Survey (NorCAS) between 1998–2010 formed this population-based register study. Information on chorionicity was obtained from the Northern Survey of Twins and Multiple Pregnancy. The percentage of live births and stillbirths in the population that occurred with CHD (either in live births, fetal deaths (≥20 weeks gestation) or terminations of pregnancy for fetal anomaly) was calculated for twins and singletons. Similarly, the percentage of twin pregnancies associated with at least one case of CHD was calculated. The RR of CHD in twins compared to singletons was estimated using Poisson regression. Cases with extra-cardiac anomalies were excluded.

Results There were 399,414 singleton births of which 2,984 (0.7%) had CHD. Among 11,871 twin births, there were 154 (1.3%) cases of CHD, meaning at least one twin was affected by CHD in 2.5% of twin pregnancies. Of 8,605 DC births and 2,317 MC births, there were 96 (1.1%) and 47 (2.0%) cases of CHD, respectively. Compared to singletons, twins were at 73% significant increased risk of CHD (RR = 1.73, 95% CI: 1.48–2.04; p < 0.001). Compared to singletons, DC twins were at 49% significant increased risk of CHD (RR = 1.49, 95% CI: 1.22–1.83; p < 0.001) and MC twins were at 172% significant increased risk of CHD (RR = 2.72, 95% CI: 2.04–3.62; p < 0.001). The risk of severe CHD in MC twins was particularly high (RR = 3.92, 95% CI: 1.25–12.30; p = 0.019).

Conclusion In 2.5% of twin pregnancies, at least one twin was affected by CHD. Compared to singletons, twins were at 73% increased risk of CHD, the risk being substantially higher amongst MC twins. The risk of severe CHD was particularly high in MC twins compared to singletons. This information is important for health professionals when counselling women with a twin pregnancy.