Monozygotic (MZ) twins concordant for cloacal dysgenesis were diagnose
d at 15 weeks' gestation on ultrasound exam. The structural alteration
s result from abnormal morphogenesis of the cloacal membrane and conti
guous anlage with resultant secondary changes. These defects are sugge
stive of teratogen exposure or an endogenous event early in gestation,
resulting in MZ twinning and the development of a polytopic field def
ect.