Fetal growth is exponential and during the last 20 weeks of gestation the fetus gains 95% of its weight. Genetic, nutritional, environmental, uteroplacental, and fetal factors have been suggested to influence fetal growth. Uteroplacental and umbilical blood flow and transplacental glucose and fetal insulin are major determinants of fetal growth. The role of the fetal pituitary (growth hormone) and thyroid in fetal growth is not well understood; human anencephalic or athyroid fetuses usually have no or only minor retardation of growth. Also, it is not clear whether placental lactogen or somatomedin or a somatostatin-like substance of the placenta and fetus influences fetal growth. From experiments on rats it may be assumed that a specific placental-fetal growth-promoting and growth-regulating factors(s) exists. Identification of such a placental-fetal growth factor(s) in humans might aid in the prevention, diagnosis, and treatment of fetal growth retardation.