Cardiac malformations are the commonest anomalies with astonishing similarity in reported birth rates in different centres. Searching for causative factors is still ongoing process. Results of Baltimore Washington Infant Study and New England Study and presents that cardiac defects are remarkably constant in time and place, which tends to argue against hidden teratogens as a major cause of malformations. Despite this, many authors have favoured a major environmental contribution on the basis of twin studies. There is well-known that cardiac defects are common in twin pregnancies, occurring more commonly in monozygotic twins and usually affecting one twin in more than 90% of cases. The mechanism for the increase malformation remains unknown, although several explanations have been proposed. One possibility is that monozygotic twinning process itself increases the incidence of congenital heart disease, with postzygotic unequal division of the inner cell mass being responsible for discordant cardiovascular anatomy. Another contributor might be phenotypic variability of the same genome or possibly even genetic inter-twin differences caused by de novo mutations.
Here we report case of monozygotic male twins discordant for cardiac malformations presenting with pulmonary atresia and intact ventricular septum (PS/IVS) in one of them. Another twin presents with subaortic ventricular septal defect (VSD) and many small muscular ventricular defects. According to the developmental-mechanistic approach this cardiac defects belong in different groups. Twin pairs with congenital heart defects generally have concordant defects by mechanistic group. To our knowledge the occurrence of PS/IVS and VSD in monozygotic twins has never been reported.