Objective: To test the hypothesis that maternal and cord serum leptin concentrations correlate with birth weight of infants. Methods: Pregnant women (n = 135) of low socioeconomic status who delivered full-term infants were selected from more than 1500 women who participated in a study to identify factors related to fetal growth restriction (FGR). They were divided into two groups based on their infants being classified as having FGR (n = 66) or not (n = 69), and each group was divided further into three subgroups based on prepregnancy body mass index (BMI): less than 19.8, 19.8-28.9, and 29 or more. Sample estimations indicated that 20 subjects per subgroup would be adequate to detect 50% difference in leptin concentrations. Results: Mean maternal serum leptin concentrations adjusted for BMI were highest at approximately 22-27 weeks' gestation (29.8 ng/mL) and declined thereafter until term (25.2 ng/mL). Leptin concentration and prepregnancy BMI correlated significantly. We found neither significant difference in leptin concentrations between the subjects with and without FGR infants nor significant correlation between maternal leptin concentrations and birth weight of infants. Mean cord serum leptin concentration (10.8 ng/mL) was lower than maternal concentrations and correlated significantly with birth weight (r = .61, P < .001). Conclusion: Our findings suggest that maternal leptin concentration during pregnancy is not an accurate indicator of fetal growth. Cord serum leptin concentrations were lower than maternal serum concentrations and correlated significantly with birth weight.