Triventricular heart with three atrioventricular valves in a conjoined twin.

An infant, who was conjoined to a partially formed twin, died from congenital cardiac disease which could not be diagnosed accurately by catheterization and cineangiography. Pathologic examination showed a triventricular heart with three atrioventricular valves. One great vessel system was present, and the aorta was transposed. The findings in this infant suggest that the incorporation of cardiac anlage from an incompletely or abortively formed fetus may explain the presence of supernumerary structures in complex congenital cardiac malformations.