Human prolactin (hPRL) is a hormone produced by the lactotropic cells of the hypophysis and by the endometrium converted to decidua in pregnancy. Prolactin of decidual origin is identical with the pituitary hormone. Decidual prolactin is released mainly by the extraplacental fetal membranes. Prolactin production was found also in the endometrium in late secretory phase of normal menstrual cycle. It was noted that hPRL production by the decidua in various periods of pregnancy is closely correlated with hPRL concentration in the amniotic fluid. Peak hPRL release into the amniotic fluid is in the 24th week of pregnancy. The most important biological function of decidual prolactin is its effect on water and electrolyte transport for the needs of the fetus, and this transport takes place mainly across the fetal membranes. Moreover, it was found that prolactin affects the synthesis of fetal surfactant, influences calcium absorption in the fetal intestine. Calcium and phosphorus are obtained by the fetus from the amniotic fluid. Many authors suggest that decidual prolactin has a role in the process of implantation and early development of the blastocyst.