Second-trimester sonographic findings of fetal trisomy may include structural abnormalities or sonographic markers of fetal aneuploidy. Unlike structural anomalies, sonographic markers of fetal aneuploidy are insignificant by themselves with regard to outcome, are nonspecific--most frequently seen in normal fetuses, and are often transient. Our objective was to review the second-trimester sonographic findings of the major trisomic conditions, trisomies 13, 18, and 21. We reviewed a number of the most commonly accepted markers, including nuchal thickening, hyperechoic bowel, echogenic intracardiac focus, renal pyelectasis, shortened extremities, mild cerebral ventricular dilatation, and choroid plexus cysts. Markers associated with trisomy 21 were emphasized. The sensitivity of sonography for detection of fetal trisomic conditions varies with the type of chromosome abnormality, gestational age at the time of sonography, reasons for referral, criteria for positive sonographic findings, and the quality of the sonography. As an estimate, 1 or more sonographic findings can be identified in approximately 90% of fetuses with trisomy 13, 80% of fetuses with trisomy 18, and 50% to 70% of fetuses with trisomy 21 (Down syndrome). The presence or absence of sonographic markers can substantially modify the risk of fetal Down syndrome and is the basis of the so-called genetic sonogram. Because maternal biochemical and sonographic markers are largely independent, combined risk estimates will result in even higher detection rates than either alone.