Leptin is a 16-kDa polypeptide, encoded in humans by the LEP gene. This protein is probably involved in the regulation of ovarian function, oocyte maturation and embryo development, and in the implantation process. During pregnancy, leptin is produced in the placenta; its values increase in the maternal blood particularly in the second trimester and decline postpartum. Leptin levels may be abnormally high in pregnancies complicated by conditions such as diabetes mellitus and pre-eclampsia. The role of leptin in fetal physiology has not been clarified, although cord blood values correlate significantly with birth weight. It is expected that further research will provide new insight into the physiological importance of leptin in the pregnant woman and fetus.