THE USE OF 2ND-TRIMESTER GENETIC SONOGRAM IN GUIDING CLINICAL MANAGEMENT OF PATIENTS AT INCREASED RISK FOR FETAL TRISOMY-21

Objective: To test the efficacy of ultrasound in detecting fetuses wit h trisomy 21. Methods: From November 1, 1999, to December 31, 1995, a second-trimester genetic sonogram was offered to all women with single ton fetuses at increased risk (at least 1:274) for trisomy 21, who had either declined genetic amniocentesis or chose to have a sonogram bef ore deciding whether to undergo an amniocentesis. In addition to stand ard fetal biometry, the following ultrasound markers for aneuploidy we re evaluated: structural anomalies (including face, hands, and cardiac [four-chamber view and outflow tracts]), short femur, short humerus, pyelectasis, nuchal fold thickening, echogenic bowel choroid plexus cy sts, hypoplastic middle phalanx of the fifth digit, wide space between the first and second toes, and two-vessel umbilical cord. Outcome inf ormation included the results of genetic amniocentesis, if performed, or the results of postnatal pediatric assessment and follow-up. Result s: Five hundred seventy-three patients had a genetic sonogram between 15 and 23 weeks' gestation: 378 patients had advanced maternal age (at least 35 years), 141 had abnormal serum biochemistry, and 54 had both . The majority (495, or 86.3%) had a normal genetic sonogram (absence of abnormal ultrasound markers); 51 (9%) had one marker present, and 2 7 (4.7%) had two or more markers present. Outcome was obtained on 422 patients (the remaining were ongoing pregnancies or were lost to follo w-up). Twelve of 14 fetuses with trisomy 21, one fetus with trisomy 13 , and one fetus with triploidy had two or more abnormal ultrasound mar kers present; one fetus with trisomy 21 had one abnormal marker and on e had a completely normal ultrasound. When one or more abnormal ultras ound markers were present, the sensitivity, specificity, and positive and negative predictive values for trisomy 21 were 92.8%, 86.7%, 19.4% , and 99.7%, respectively. When two or more abnormal ultrasound marker s were present, the corresponding values were 85.7%, 96.8%, 48%, and 9 9.5%. In the study population, the amniocentesis rate was 12.7% overal l and 17.3% in cases with known outcome. Conclusion: Second-trimester genetic sonogram may be a reasonable alternative for patients at incre ased risk for fetal trisomy 21 who wish to avoid amniocentesis. In exp erienced hands, this approach may result in a high detection rate of t risomy 21 (93%), with an amniocentesis rate of less than 20%.