Estrogens have profound effects on bone- and lipid homeostasis, modulate immune functions and play a major role in the regulation of the development and function of central nervous system, cardiovascular system and respiratory system. They induce hepatic protein production. There is a strong connection between estrogens and several hormone systems including growth hormone--IGF axis and renin-angiotensin system. The extragenital effects of estrogens are probably more evident in the fetus than in fertile women. However, direct data are not available because of the lack of estrogen-deficient pregnancy models. Theoretically, in immature infants the premature cessation of in utero high estrogen levels is associated with the progression of perinatal acute complications. The volume of uterus in female premature infants is lower than in term newborns, the plasma levels of atherogen lipids are high, osteopenia due to inadequate bone acquisition is frequent. Inflammatory complications are common. Recently, the physiological effects of estrogen- and progesterone supplementation of premature infants have been evaluated in a pilot study. The preliminary results indicate, that the risk of bronchopulmonary dysplasia, osteopenia might be decreased in treated infants. Further studies are needed to confirm the effectivity and long term safety of estrogen supplementation of preterm neonates.