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Objective: To summarize by meta-analysis the accumulated data on the screening performance of second-trimester sonographic markers for fetal trisomy 21., Methods: We conducted a literature search to identify studies between 1995 and September 2012 that provided data on the incidence of sonographic markers in trisomy 21 and euploid fetuses at 14-24 weeks' gestation. Weighted independent estimates of detection rate, false-positive rate and positive and negative likelihood ratios (LR) of markers were calculated., Results: A total of 48 studies was included in the analysis. The pooled estimates of positive and negative LR were, respectively: 5.83 (95% CI, 5.02-6.77) and 0.80 (95% CI, 0.75-0.86) for intracardiac echogenic focus; 27.52 (95% CI, 13.61-55.68) and 0.94 (95% CI, 0.91-0.98) for ventriculomegaly; 23.30 (95% CI, 14.35-37.83) and 0.80 (95% CI, 0.74-0.85) for increased nuchal fold; 11.44 (95% CI, 9.05-14.47) and 0.90 (95% CI, 0.86-0.94) for hyperechogenic bowel; 7.63 (95% CI, 6.11-9.51) and 0.92 (95% CI, 0.89-0.96) for mild hydronephrosis; 3.72 (95% CI, 2.79-4.97) and 0.80 (95% CI, 0.73-0.88) for short femur; 4.81 (95% CI, 3.49-6.62) and 0.74 (95% CI, 0.63-0.88) for short humerus; 21.48 (95% CI, 11.48-40.19) and 0.71 (95% CI, 0.57-0.88) for aberrant right subclavian artery (ARSA); and 23.27 (95% CI, 14.23-38.06) and 0.46 (95% CI, 0.36-0.58) for absent or hypoplastic nasal bone. The combined negative LR, obtained by multiplying the values of individual markers, was 0.13 (95% CI, 0.05-0.29) when short femur but not short humerus was included and 0.12 (95% CI, 0.06-0.29) when short humerus but not short femur was included., Conclusion: The presence of sonographic markers increases, and absence of such markers decreases, the risk for trisomy 21. In the case of most isolated markers there is only a small effect on modifying the pre-test odds for trisomy 21, but with ventriculomegaly, nuchal fold thickness and ARSA there is a 3-4-fold increase in risk and with hypoplastic nasal bone a 6-7-fold increase., (Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.)