Survival and development of the mammalian conceptus depends on a variety of factors. Fetal growth is controlled by genetic and environmental determinants that may limit the mother's capacity to provide an appropriate environment (e.g., space, nutrients, temperature). Exchanges between the mother and fetus take place within the placenta. Interestingly, despite the diversity of mammalian species in terms of placental structure and hormonal functions, placental size at term always correlates with birth weight, reflecting the essential role of this temporary organ. The placenta is the site of major endocrine activity, including synthesis of a broad range of steroid and peptide hormones, growth factors, cytokines, and other bioactive factors. Some of these are produced exclusively by the placenta, including chorionic gonadotropin, and growth hormone (GH)/prolactin-like hormones. This article focuses on the expression, regulation, and physiologic role of placental GHs in mammalian species. Published data suggest that placental GHs are essential for adapting the maternal metabolism to pregnancy, for normal placental development, and therefore for fetal growth.