We evaluated 406 women with late third-trimester ultrasound examinations to determine whether the sonographic diagnosis of a large for gestational age (LGA) fetus, defined as an estimated fetal weight at or above the 90th percentile, altered the management of labor and delivery. The sonographic prediction of LGA fetuses had a sensitivity, specificity, and positive predictive value of 50, 90, and 52%, respectively. Women without the sonographic diagnosis of an LGA fetus (N = 338) differed from those with the diagnosis (N = 68) in the frequency of diagnosed labor abnormalities (19 versus 30%, P = .03), use of epidural anesthesia (57 versus 74%, P = .01), and the incidence of cesarean deliveries (32 versus 53%, P = .004). To determine whether it was the sonographic prediction of an LGA fetus or the actual fetal weight that altered clinical management and perinatal outcomes, we stratified the study population into four groups and compared the true negatives with the false positives and the false negatives with the true positives. The incorrect sonographic diagnosis of an LGA fetus had a statistically significant effect on both the diagnosis of labor abnormalities (P = .04) and the incidence of elective cesareans (P = .04) in pregnancies with appropriate for gestational age birth weights.