Monozygous twin with polymicrogyria and normal co-twin.

Monozygous twinning has been associated with brain lesions such as holoprosencephaly, hydranencephaly, multicystic encephalomalacia, and porencephaly. Polymicrogyria is rare but has been reported in autopsied cases. We present a monozygotic twin with complicating twin-twin transfusion syndrome, the donor exhibiting normal psychomotor development and the recipient psychomotor delay associated with the brain anomaly, polymicrogyria. The brain anomaly might be due to blood pressure instability or a circulatory disturbance resulting from increased coagulability due to polycythemia.