OBJECTIVE To assess the validity of ultrasound estimation of fetal weight at term. METHODS We conducted a prospective observational study whereby all ultrasonic biometric measurements were done by a single observer. Fifty pregnant women at term had ultrasonic measurement of various fetal biometric parameters performed within a week of delivery. Fetal weight was estimated by the use of four reported methods (Aoki, Campbell, Shepard, and Hadlock formulas). We compared estimated weight with the birth weight after the estimated fetal weight was adjusted by adding 25 g for each day between the ultrasound measurements and delivery. RESULTS The adjusted estimated fetal weight obtained from all four formulas tended to be lower than measured birth weight. The smallest mean difference was obtained with the Shepard and Aoki formulas (51.4 g and 60.5 g, respectively), whereas the Campbell and Hadlock formulas produced larger mean differences (141.8 g and 190.7 g, respectively). The Aoki formula generated the smallest range between the limits of agreement (-324.2 to 445.2 g) whereas the Campbell formula produced the largest range (-286.5 to 570.1 g). The range between the limits of agreement generated with the Shepard and Hadlock formulas were intermediate between those produced by the Aoki and Campbell formulas. The intraclass correlation coefficients generated with the Aoki and Shepard formulas were identical (0.90). The intraclass correlation coefficients obtained with the Hadlock (0.84) and Campbell formulas (0.85) were lower. CONCLUSIONS The validity of ultrasonic estimation of fetal weight at term with all four formulas was high.