To investigate the usefulness of screening in low-risk populations, the authors evaluated the yield of ultrasonography for detecting abnormalities in 678 clinically uncomplicated pregnancies. The yield of ultrasonography in high-risk women who were referred for amniocentesis was remarkably similar to the yield in other women. All four diagnoses of twins were correct, but overall only six of 12 initial ultrasound diagnoses of fetal demise or fetal anomalies were confirmed at delivery. Of the eight major fetal anomalies present at delivery, including two cases of Down's syndrome, three had been detected by ultrasonography; none of the nine minor anomalies had been detected, usually because they were too small or might be detectable only at a later gestational age. Although ultrasonography may have a nearly perfect predictive value for certain anomalies, on average, in this study, positive ultrasonography increased the probability of an adverse outcome of pregnancy from 5.3% to 36%, while a normal ultrasound examination decreased the probability to 4.4%. These data, which emphasize the implications of screening a low-risk population, suggest that recommendations regarding routine screening obstetrical ultrasonography should await sufficiently large controlled trials demonstrating consistent clinical benefit, in terms of reassurance or of providing a baseline for future comparison or in terms of improved outcome at a reasonable cost.