ENLARGED CISTERNA-MAGNA IN THE 3RD-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 enlar ged cisterna magna (5 males vs. 1 female), indicating males are more l ikely to have this associated defect, Each of the 6 cases was associat ed with other trisomy markers including intrauterine growth retardatio n and polyhydramnios. Since a significant proportion of pregnancies ma y escape early prenatal care and some potentially detectable fetal abn ormalities may be missed on early ultrasound and/or by maternal serum screenings, prenatal detection of an enlarged cisterna magna associate d with intrauterine growth retardation and/or polyhydramnios in late g estation should prompt complete genetic counseling, rapid karyotyping and a careful search for other congenital anomalies.