Introduction: Leptin is a polypeptide hormone and in pregnancy it is secreted by the placenta and maternal and fetal adipose tissues. The expression of leptin and its specific receptors is observed in the uterine endometrium which indicates leptin involvement in the implantation process and embryonic/fetal development. Normal leptin production is a factor responsible for uncomplicated gestation, embryo development and fetal growth. Objective: To compare at delivery maternal serum and cord blood leptin concentrations in normal pregnancies and in pregnancies complicated by intrauterine growth restriction (IUGR). Material and methods: The study was performed in 25 pregnant women with isolated IUGR diagnosed by ultrasonography (study subjects) and in 194 pregnant women without any comorbid health conditions (controls). Leptin concentrations in maternal serum and in cord blood samples collected at delivery were measured by ELISA and subsequently analyzed by maternal Body Mass Index (BMI), mode of delivery, and infant gender and birth weight. For comparative analyses of normally distributed variables, parametric tests were used, i.e. the Student-t to test the assumption of homogeneity or non-homogeneity of variance and a One-Way ANOVA when more than two groups were compared. The non-parametric Mann-Whitney test was used when the distribution was not normal. The Pearson correlation coefficient was calculated to assess the correlation between normally distributed variables (p<0.05). Results: In pregnancies complicated by IUGR, the mean maternal serum leptin concentration at delivery was significantly higher (52.73 ± 30.49 ng/mL) than in normal pregnancies (37.17 ± 28.07 ng/mL) (p=0.01). The mean cord blood leptin concentration in pregnancies complicated by IUGR was 7.97 ± 4.46 ng/mL and significantly lower than in normal pregnancies (14.78 ± 15.97 ng/mL) (p=0.04). In normal pregnancies, but not in pregnancies complicated by IUGR, a statistically significant correlation was established between maternal serum leptin concentrations and maternal BMI at delivery (r=0,22; p=0.00). No statistically significant correlation was found between cord blood leptin concentrations and maternal BMI in either study subjects or controls. In normal pregnancies, but not in pregnancies complicated by IUGR, a strong correlation was observed between cord blood leptin concentrations and birth weight (r=0,23; p=0.00). In both study subjects and controls, there were no correlations between leptin concentrations in maternal serum and cord blood and infant gender and mode of delivery. Conclusions: Elevated maternal blood leptin concentrations in pregnancies complicated by IUGR may indicate a significant adverse effect of elevated leptin on fetal growth. Enhanced leptin production by the placenta suggests leptin as a candidate marker of placental insufficiency. The differences in leptin concentrations, measured in maternal serum and in cord blood, between the study subjects and controls suggest that deregulated leptin levels may increase the risk of obstetric complications associated with placental insufficiency.