OBJECTIVE To investigate normal pregnancies to determine whether there is a relationship between umbilical resistance and fetal growth. METHODS This prospective study was conducted in three academic departments. Third trimester routine prenatal ultrasonography was used to estimate fetal weight and measure umbilical resistance index (RI). After delivery the birthweight was noted, along with the time interval between the ultrasound and the delivery, and then the weekly weight gain was calculated. These data were then used to determine the relationship between fetal growth and umbilical artery RI. RESULTS Mean patient age was 32 ± 4.8 years; mean RI was 0.62 ± 0.07, and mean weight gain was 186.4 ± 63.9 g/week. Mean percentage weight gain relative to the estimated weight on third trimester ultrasound was 8.86 ± 3.8% per week. There was an inverse linear relationship between umbilical artery RI and fetal growth: percentage of weight gained per week relative to the estimated weight during third trimester ultrasound (%/week) = [31.3 - (36.1 × RI)] × 100. CONCLUSIONS In normal pregnancies there seems to be a linear relationship between umbilical RI measured at 31-34 weeks of gestation and average fetal growth in the third trimester. The greater the resistance index, the lower the weight gain.