Abnormal umbilical vessels and systemic circulatory reversal in thoracopagus twins

A human thoracopagus of slightly unequal twins having a common umbilical cord with exomphalos is described with special reference to the cardiovascular system. The larger twin had single umbilical artery. The smaller twin had a large left umbilical artery which was in direct continuity with the aorta, a small right umbilical artery with sole connection to the right external and internal iliac arteries (right common iliac artery was absent), missing umbilical vein, a rudimentary nonfunctional heart with atresia of the truncus arteriosus, and a right-sided aortic arch. Exomphalos of both twins is accounted by the possible defective growth of umbilical mesoderm caused by abnormal umbilical vessels. Reversal of sustemic circulation in the upper part of the body of small twin is discussed. The importance of a careful study of umbilical veins is emphasized.