Prenatal detection of trisomy 18: Accuracy of targeted ultrasound

The purpose of this study is to determine the accuracy of sonographic detection of trisomy 18 at 10-22 weeks gestation in a large multicenter prenatal population. Previous reports on this topic have potential limitations of small numbers, and findings that may reflect the experience of only one institution. We reviewed the cytogenetic databases at our institutions from 1996-2006 and evaluated all cases of trisomy 18. The ultrasound reports and clinical history of each patient were reviewed, and individual anatomic findings were recorded. IUGR was considered present if any growth parameter (BPD, AC, FL) was <5%ile. Ultrasound was defined as abnormal if a fetal anomaly and/or IUGR was noted in the original study (prior to the knowledge of abnormal karyotype). A total of 79 fetuses with trisomy 18 had ultrasounds performed at 10-22 weeks gestation in our perinatal centers. Seven had only first trimester scans, and 72 had second trimester ultrasound. The overall accuracy rate for detection of trisomy 18 in the second trimester was 94.4% (68 of 72). IUGR with anomalies were present in 35 cases (49%), anomalies alone were noted in 30 cases (42%) and IUGR alone in 3 (4%). The most common anomalies found are listed in the table. Three of the 4 cases missed had a gestational age of 17 weeks or less. There was only 1 case where the diagnosis was missed after 18 weeks. Second trimester targeted ultrasound has high accuracy in detection of trisomy 18. The ultrasound study should focus not only on a careful search for anomalies, but also on detection of abnormal fetal growth. In patients with increased risk of trisomy 18, this information can be used to reduce risk and assist with decision making for invasive prenatal testing.Tabled 1Abnormal second trimester findings (N=72)CardiacHandsCNSAbd Wall2V CordN=35N=28N=17N=14N=12 Open table in a new tab