Congenital cardiac abnormalities in monozygotic twins. Report and review of the literature.

A pair of monozygotic twin girls is reported with concordance for 3 congenital cardiac abnormalities: (1) secundum atrial septal defect, (2) aneurysm of the membraneous ventricular septum, and (3) electrocardiographic frontal plane left axis deviation. A review of the published materials shows a 9.5 per cent incidence of concordance for congenital heart disease among monozygotic twins. In those in whom a precise cardiological diagnosis was made, 15/16 pairs (95%) were concordant for a specific defect, 2 had an additional defect, and only 1 pair had completely dissimilar defects. Concordance for congenital heart disease in monozygotic twins is uncommon, but when it occurs the defects will most often be identical.