Prenatal growth is under genetic, environmental and endocrine control. The maternal environment intervenes through physical constraints and through the transplacental transfer of oxygen and nutrients, that may be influenced. by maternal serum levels of insulin-like growth factor I (IGF-I). The placenta is crucial for fetal growth. Two hormones tonically secreted by the placenta, placental growth hormone and placental lactogen, appear to participate in maternal metabolism and IGF production, and thus in fetal growth. Fetal insulin and IGFs emerge as the principal endocrine regulators of prenatal growth, whereas fetal pituitary growth hormone plays a minor role. Glucocorticoids and thyroid hormones accelerate the velocity of fetal maturation. In case of threatening premature birth, glucocorticoids and thyroid stimulating hormone-releasing hormone (TRH) can be administered to accelerate fetal maturation, in an attempt to prevent neonatal morbidity.