ObjectiveTo investigate which second-trimester ultrasound markers for aneuploidy are the most diagnostically efficient in detecting fetal trisomy 21.MethodsAll second-trimester genetic sonograms performed since November 1, 1992 for women at increased risk for fetal trisomy 21 were analyzed restrospectively. Statistical analysis included descriptive statistics, the test of proportions, and univariate and multivariable logistic regression analysis using trisomy 21 as the dependent variable and ten aneuploidy ultrasound markers as independent variables.ResultsThere were 581 normal fetuses, 23 with trisomy 21 and four with other chromosomal abnormalities. When one or more abnormal ultrasound markers were present, the sensitivity and flase-positive rate for trisomy 21 were 87% and 13.4%, respectively. After adjusting for confounders, multivariate logistic regression analysis showed the best combination of ultrasound markers for detecting trisomy 21 to be nuchal fold thickening (relative risk [RR] 85.5; 95% CONFIDENCE INTERVAL [CI] 20.4; 95% CI 4.5, 92.1)). The model combining these three ultrasound markers yielded a sensitivity of 87% and a false-positive rate of 6.7%.ConclusionBy using only three ultrasound markers (combination of nuchal fold thickening, pyelectasis, and short humerus) the false-positive rate is decreased from 13.4% to 6.7% without any compromise in the sensitivity (87%). The clinical usefulness of evaluating the various secondtrimester ultrasound markers needs to be evaluated in prospective studies.