Background:Leptin, the protein product of the ob gene, appears to function as a link between adiposity, satiety, and activity. Leptin has been found to vary with Tanner stage independent of adiposity and found to be necessary for pubertal development, conception, and pregnancy in mice. These findings suggest a role for leptin in childhood growth and development. Objective:(1)To investigate mother/newborn (m/n) pairs to determine the role of leptin in neonatal development. (2)To investigate the role of the placenta as a source of possible leptin production in utero. Design: Cross-sectional study of 100 consecutive m/n pairs for measurement of maternal(venous) and newborn (cord blood) leptin, insulin, and glucose, BMI (body mass index), and arm fat. Placental tissue was obtained from five mothers and assayed for leptin mRNA by RT/PCR. Human placental cell lines JAR and JEG-3 were assayed for leptin mRNA expression. Results:Leptin concentrations in cord blood correlated with newborn weight, and arm fat, but not with insulin or glucose. Maternal leptin concentrations correlated with preprenancy BMI, BMI at delivery, arm fat, and insulin. There was no correlation between maternal and newborn leptin concentration. Placental tissue from five separate placentas expressed leptin mRNA by RT/PCR analysis. Two human trophoblastic placental cell lines, JAR and JEG-3, also expressed leptin mRNA. In placental tissue, the range of leptin mRNA expression was equal or in increased amounts compared to adipose tissue. Conclusions: There was no significant association between serum leptin concentrations in 100 m/n pairs. Thirteen percent of newborns had higher leptin concentrations than did their mothers, suggesting a source of leptin in the newborn independent of adipose tissue. Leptin mRNA was present in placental tissue and in JAR and JEG-3 placental cell lines in quantities comparable to those found in adipose tissue. These results indicate a potential additional role for leptin as a placental growth hormone necessary for intrauterine growth and neonatal development.