A prospective study was undertaken to examine whether Doppler velocimetry studies of the umbilical artery give a better or an earlier prediction of intrauterine growth retardation (IUGR) than do sonographic estimation of fetal weight. A total of 385 examinations were performed between 30 and 42 weeks of gestation on 168 patients who were at risk for IUGR. Forty-two (25%) of the patients delivered an infant with a birth weight below the tenth percentile. Although sensitivity was lower for the systolic to diastolic ratio (55%) of the umbilical artery than for the sonographic estimation of fetal weight (76%), the umbilical artery studies had a higher specificity (92% versus 80%) and predictive value of a positive test (73% versus 58%) when the last study to delivery interval was within 2 weeks. Furthermore, among 21 IUGR pregnancies with serial studies, the umbilical systolic to diastolic ratio was abnormal at a significantly earlier gestational age than when the sonographic estimate of fetal weight identified growth retardation. These findings indicate that sonographic biometry is a more sensitive technique for identifying IUGR but that the umbilical artery waveforms are a valuable adjunct for improving the diagnostic accuracy for the prenatal detection of IUGR. These data also provide suggestive evidence that umbilical artery velocimetry may be predictive of IUGR at an earlier gestational age than sonographic estimation of fetal weight.