Second trimester ultrasonography may identify 77 to 97% of fetuses with trisomy 18.

Between 1990 and 1999, 30 second trimester fetuses with trisomy 18 and 2000 control fetuses underwent real-time and color Doppler ultrasonographic examination followed by genetic amniocentesis. Abnormal fetal anatomy was present in 97% of fetuses with trisomy 18, with a false-positive rate of 15.7%. Logistic regression identified six markers (choroid plexus cysts, central nervous system malformations, an abnormal nuchal skin fold, ventricular septal defect, outflow tract abnormalities of the heart, and right-to-left chamber disproportion of the heart) and one interaction between markers (right-to-left chamber disproportion and outflow tract abnormalities) to significantly contribute to the identification of 93% of fetuses with trisomy 18, with a false-positive rate of 8.9%. Noncardiovascular markers (choroid plexus cysts, central nervous system malformations, and abnormal nuchal skin fold) identified 77% of fetuses with trisomy 18, with a false-positive rate of 3.9%. Combining right-to-left chamber disproportion of the heart with choroid plexus cysts, central nervous system malformations, and nuchal skin folds identified 83% of fetuses with trisomy 18, with a false-positive rate of 4.4%.