Prenatal diagnosis of trisomy 9 mosaic presenting as a case of Dandy-Walker malformation.

Trisomy 9 syndrome is a rare chromosomal anomaly associated with specific patterns of multisystem dysmorphism and occasional central nervous system (CNS) malformations, the most common being the Dandy-Walker malformation. Milder anomalies are usually seen with trisomy 9 mosaicism. We report what we believe to be the first case of a baby with an isolated Dandy-Walker malformation which was diagnosed prenatally and was subsequently found to have a trisomy 9 mosaic syndrome.