Leptin is a polypeptide hormone, the product of the ob gene that works as adipostat in the human organism. Leptin is mainly produced in adipose tissue, but its presence can be found in smalleramounts in other tissues too. Leptin signalises the available amounts of energy and influences growth, development, reproduction and metabolism. It works through leptin receptors, OB- Rb long form of protein and OB-Rb short form. Leptin is a metabolic signal that signalises the readiness of the organism for reproduction. It also takes part in controlling sexual differentiation of the brain, the beginning of puberty, the pregnancy through placenta and in lactation. After the puberty and in the early stage of pregnancy, the concentration of leptin in serum grows, and it reduces with the end of pregnancy and menopause. Leptin receptors are present in the endometrium and decidua and can participate in placentalimplatation of blastocyst. During pregnancy, leptin is produced by mothers, fetus and placenta. It is also present in the blood of an infant, and influences the infant's length and mass, fetal angiogenesis, hematopoiesis of embryos, and biosynthesis of hormones between the mother and the placenta. New researches point to a connection between insulin and leptin, which leads to fetal overgrowth in mothers diagnosed with diabetes.