Trisomy 18 in monozygotic twins.

We report a very rare case of a pair of monozygotic twins with trisomy 18 discordant for major anomalies. Our case contributes to published data on the role of nongenetic factors in the etiology of discordance of congenital malformations in genotypically identical twins. We stress the importance of accurate ultrasonic examination in reducing the number of caesarean sections to deliver trisomy 18 infants.