This study investigates the relationship between leptin and fetal bone metabolism by measuring fetal blood levels of leptin, carboxy-terminal pro-peptide of type I pro-collagen (PICP; a marker of bone formation) and cross-linked carboxy-terminal telopeptide of type I collagen (ICTP; a marker of bone resorption). The median gestational age at the time of sampling was 23 weeks (range, 18-35). There was a positive correlation between leptin concentration and gestational age (r = 0.543, P < 0.001) and a negative correlation between both PICP and ICTP and gestational age (r = -0.592 and r = -0.550, respectively, and P < 0.001 for both). Also, there was a negative correlation between the concentrations of leptin and both PICP (r = -0.260, P = 0.022) and ICTP (r = -0.622, P < 0.001). Using multiple regression analysis, fetal leptin concentration was positively correlated to the gestational age (r = 0.240, P = 0.042) and negatively correlated to ICTP (r = -0.420, P = 0.001). The increase in leptin concentration with gestational age is consistent with adipose tissue development and the subsequent accumulation of fat mass. The negative correlation between fetal leptin and ICTP suggests that leptin may decrease bone resorption with the overall effect of increasing bone mass. Therefore, leptin may play a role in fetal bone metabolism as part of its effect on fetal growth and development.