Report on the 11- to 13+6-week ultrasound evaluation as a screening test for trisomy 21 in singleton pregnancies.

We sought to update the value of the 11- to 13 (+6)-week ultrasound evaluation for prenatal screening of trisomy 21. An English literature search was conducted for pertinent original research articles, review articles, and websites related to the 11- to 13 (+6)-week ultrasound evaluation. Reports from prenatal diagnostic units published since 1990 that included two- and three-dimensional ultrasound and Doppler hemodynamic evaluations for prenatal screening of trisomy 21 between weeks 10 and 13 (+6) of gestation were selected. Alternative screening methods were identified, and clinical information concerning each of them was extracted, integrated into tables, and then synthesized to provide meaningful clinical interpretations. Finally, mechanisms that could lead to improvements in the efficiency of these screening tests are discussed. Two-dimensional measurements of the fetal nuchal translucency play a key role in screening for trisomy 21. Evaluations of the nasal bone, ductus venosus, tricuspid valve function, frontomaxillary facial angle, hyperechogenic bowel, intracardiac echogenic focus, and renal pelvis fullness can become part of the 11- to 13 (+6)-week screening test if the imaging protocols are standardized. The role of 3-D ultrasound during the 11- to 13 (+6)-week ultrasound evaluations is under investigation.