Twins, Conjoined Twins, and Cardiac Defects

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Abstract
Human twins have always been of special interest to pediatricians and others concerned with congenital malformations. Monozygous twins, which comprise 30% of all twins, arise from a single ovum and share identical genetic material, while dizygous twins arise from separate ova, each with a distinct genetic composition. A study of cardiac malformations in twins should provide some insight into the causation of congenital heart disease. Dr Ray Anderson1 recently reviewed his experience in 109 sets of twins and triplets with a variety of cardiac defects. His study, which comprises the largest number of random cases so far published, revealed a concordance rate for cardiac defects of 8.2% in monozygous twins and 2.2% in dizygous twins. In an earlier but smaller study, I found similar results.2 When I pooled the twin studies of Uchida and Rowe,3 Ross,4 and my own patients, a concordance rate of 8.8% was