Sonographic findings of trisomy 18 in the second trimester of pregnancy

Objective. The purpose of this study was to examine the sonographic findings in fetuses with trisomy 18 in the second trimester of pregnancy. Methods. A retrospective review of the cytogenetic laboratory databases at 6 tertiary referral centers identified all cases of trisomy 18. The prenatal sonographic studies in fetuses at 15 to 21 weeks' gestation, done before invasive testing for the karyotype, were reviewed for anatomic and biometric findings. We defined abnormal fetal biometric findings as a biometric measurement (biparietal diameter, abdominal circumference, or femur length) below the fifth percentile in the second trimester. Results. Of 98 fetuses with trisomy 18, 95 (97%) were detected sonographically; an anomaly was found in 92 (94%). A biometric measurement below the fifth percentile was noted in 50 (51 %). Cardiac (63%) and central nervous system (34%) anomalies were most frequently detected. Although choroid plexus cysts were commonly seen, no fetuses with trisomy 18 and isolated choroid plexus cysts were found. Conclusions. Targeted sonography identified abnormal fetal anatomy or abnormal biometric findings in 97% of fetuses with trisomy 18 in the second trimester. A biometric measurement below the fifth percentile was noted in half of the cases in the second trimester.