This review summarizes the experimental evidence supporting the concept that oestrogen has a central integrative role in modulating the communication that occurs between the placenta and the fetus which results in primate fetal-placental development. Thus oestrogen, acting within placental trophoblasts, regulates the functional differentiation of syncytiotrophoblasts, manifested as an upregulation of key components of the progesterone biosynthetic pathway and the 11beta-hydroxysteroid dehydrogenase (11beta-HSD)-1 and -2 enzymes controlling cortisol-cortisone interconversion. The increase in 11beta-HSD expression results in the switch in the qualitative and quantitative patterns of transplacental corticosteroid metabolism that induces maturation of the primate fetal hypothalamic pituitary adrenocortical axis. The studies outlined in this review, therefore, provide new insight into the role that oestrogen plays during the course of primate pregnancy and demonstrate that an oestrogen-dependent signalling system exists in utero that coordinates the placental and fetal dialogue critical to development of the placenta and endocrine systems underlying neonatal self-sufficiency.