Enlarged cisterna magna in the third trimester as a clue to fetal trisomy 18

Six of 19 fetuses with trisomy 18 confirmed by late karyotyping in the third trimester were found to have an enlarged cisterna magna (10 mm or more). For trisomy 18 conceptuses, there is a significant excess of females in both fetuses and livebirths; however, we found this ratio to be reversed in the third-trimester trisomy 18 fetuses with an enlarged cisterna magna (5 males vs, 1 female), indicating males are more likely to have this associated defect. Each of the 6 cases was associated with other trisomy markers including intrauterine growth retardation and polyhydramnios. Since a significant proportion of pregnancies may escape early prenatal care and some potentially detectable fetal abnormalities may be missed on early ultrasound and/or by maternal serum screenings, prenatal detection of an enlarged cisterna magna associated with intrauterine growth retardation and/or polyhydramnios in late gestation should prompt complete genetic counseling, rapid karyotyping and a careful search for other congenital anomalies.