Increased prevalence of congenital heart defects in monozygotic and dizygotic twins.

BACKGROUND Smaller studies and many case series reports indicate that congenital heart defects may be more common in monochorionic twins than in dichorionic twins and singletons. METHODS AND RESULTS We investigated congenital heart defect occurrence in all twins and 5% of all singletons born in Denmark in 1977-2001 and followed through 2006 by linking the Danish Twin Registry and Statistics Denmark registers including the National Medical Birth Register and the Danish National Patient Register. Among 41 525 twin individuals, a total of 584 twins (1.4%) had a congenital heart defect registered in the Danish National Patient Register, whereas the corresponding numbers for singletons were 648 of 74 473 (0.87%; P<0.001; ie, a 63% [95% confidence interval, 45-82%] increased risk for congenital heart defect for twins). Patent ductus arteriosus and coarctation of aorta occurred >3 times as often in twins as in singletons, at 3.9 (95% confidence interval, 2.6-5.8) and 3.1 (95% confidence interval, 1.5-6.4), respectively. The increased occurrence in twins was also found in sensitivity analyses including only inpatients or only surgically treated cases regardless of whether preterm patent ductus arteriosus was included. We were not able to demonstrate a higher risk for congenital heart defect among monozygotic twins compared with dizygotic twins, and the congenital heart defect occurrence was also increased in dizygotic twins, who are all dichorionic. CONCLUSIONS Congenital heart defect is more common in twins than in singletons, and the increased occurrence is not restricted to monochorionic twins. Intrauterine surveillance and a postnatal comprehensive cardiac assessment for both twins may be considered regardless of chorionicity and zygosity.