third ventricle enlargement among newborn infants with trisomy 21 objective our goal was to determine whether the third ventricle is significantly enlarged among neonates with trisomy 21 compared with infants without clinical signs of trisomy 21 this enlargement might be related to hypoplasia of the structures surrounding the third ventricle these structures participate in cognitive development and hypoplasia in this area may be responsible for some of the unique cognitive abnormalities observed among children with trisomy 21methods measurements of routine head sonographic scans of 57 term infants with trisomy 21 who were born between january 2000 and august 2005 were performed within 7 days after birth and were compared with measurements of head sonographic scans of 21 randomly selected healthy term infants without trisomy 21results although the 2 groups were of similar gestational ages infants with trisomy 21 were generally smaller with smaller head circumferences despite the smaller overall head circumference both the width and length of the third ventricle were enlarged among the infants with trisomy 21 vertical measurements of the lateral ventricles were similar for the 2 groupsconclusions the third ventricle is an important diencephalic space in our neonatal population we did show significant enlargement of both the length and width of the third ventricle among the infants with trisomy 21 compared with the control group of unaffected newborns we suspect that the unique neuropsychological development and cognitive dysfunction associated with trisomy 21 and the enlargement of the third ventricle in the neonatal period might be related