Vascular etiology of disruptive structural defects in monozygotic twins.

Two infants with structural defects previously undescribed in the survivor of a monozygotic twin pair are reported. One infant had hydranencephaly and a spinal cord transection, with an associated dead monozygotic co-twin of 24 weeks gestation; the other child had complete atresia of the colon and a horseshoe kidney, with a deceased co-twin of approximately six weeks gestation. These defects are presumed to be the result of in utero disruption of previously normally formed structures. They occur secondary to vascular exchange from a dead to a living monozygotic twin through placental vascular anastomoses. As illustrated by the two children described, the nature of the vascular defects seen in the survivor of a monozygotic twin pair depends on the time during gestation at which the co-twin dies. Recognition of the disruptive vascular etiology of the structural defects outlined in this report will allow for appropriate counseling with respect to the negligible recurrence risk for similar vascular accidents.