Leptin is secreted by adipose tissue and expressed in several organs, including the placenta. We investigated the influence of leptin in maternal blood on leptin expression and concentration in the placenta and umbilical cord and its influence on fetal weight and conditions at the time of birth. Twenty-five parturients were recruited, and after childbirth, placental tissue fragments, umbilical arteries, and vein blood were collected. According to leptin concentration in maternal peripheral blood, parturients were divided into low and high leptin groups. Mothers in the high-leptin group had higher age, pre-pregnancy weight, pre-pregnancy BMI, and weight in late pregnancy than mothers in the low-leptin group. Children of mothers in the high-leptin group presented an Apgar score modestly lower than those of the low-leptin group. No differences were observed in leptin concentrations in the umbilical artery and vein. However, the leptin retained in the fetus increased in the high-leptin group compared to the low-leptin group. We observed a negative correlation between fetal leptin retained and maternal age in the low-leptin group. In the high-leptin group, placental leptin concentration was positively correlated with maternal weight in late pregnancy. Nonetheless, the expression of the leptin receptor (Ob-Rb) was negatively correlated with newborn length. In conclusion, this study found a positive correlation between maternal leptin levels and maternal pre-pregnancy weight, BMI, and fetal birth weight. These findings suggest a potential role of leptin in maternal weight regulation during pregnancy and fetal growth.