Monozygotic twins with trisomy 21 and partial agenesis of the corpus callosum.

Trisomy 21 is the most common viable trisomy. Although it is invariably associated with mild to severe developmental delay and intellectual disability, no gross central nervous system malformation has been consistently identified in individuals with trisomy 21. We present the case of a monozygotic twin pregnancy in which both fetuses were identified as having trisomy 21 and partial agenesis of the corpus callosum. We discuss this rare association in the context of an emerging understanding of the neurobiology of trisomy 21.