Fetal choroid plexus cysts: a prospective study and review of the literature. To determine the incidence and significance of fetal choroid plexus cysts, a prospective study was carried out at Queen's Medical Centre, Nottingham. The incidence of cysts was found to be 0.42% and the size of cysts varied from 3 to 13 mm with a mean diameter of 6.2 mm. Choroid plexus cysts were associated with other fetal abnormalities in three cases, two of which were subsequently found to have a chromosomal abnormality, one Trisomy 18 and one Trisomy 21. A review of the reported cases of choroid plexus cysts reveals an overall incidence of 5.8% for Trisomy 18. We therefore recommend that in patients with choroid plexus cysts a detailed scan should be carried out to look for the ultrasound markers of Trisomy 18. If there is an associated abnormality or cysts are large (13 mm or greater in diameter) and bilateral then a karyotyping should be carried out. 