Hydranencephaly owing to twin-twin transfusion: serial fetal ultrasonography and magnetic resonance imaging findings.

We report a newborn girl with hydranencephaly. In the setting of a monochorionic twin pregnancy, one twin's demise was detected by ultrasonography at 18 weeks of gestation, apparently the result of a twin-twin transfusion. In the surviving twin, the evolution of ventriculomegaly, first noted at 18 weeks, to hydranencephaly at 27 weeks is documented by serial sonograms. These findings were confirmed with fetal and postnatal magnetic resonance imaging.