It is proposed that in sheep, during the last third of gestation, a progressive increase in the production of prostaglandin E2 (PGE2) by the fetal trophoblast activates the fetal hypothalamic-pituitary-adrenal axis leading to an increase in fetal cortisol concentrations. High cortisol levels increase placental 17 alpha-hydroxylase activity causing a decrease in maternal progesterone concentrations and an increase in oestrogen concentrations. The increase in the E/P ratio results in an increase in PGF2 alpha release from the maternal placenta, increased uterine activity and parturition. The increased production of PGE2 by the placenta has been linked to the rapid increase in the growth of the fetus from day 110 of gestation onwards. It is speculated that the growth pattern of the fetus represents a genetically programmed 'clock' which acts by stimulating placental PGE2 production leading to maturation of key organ systems in the fetus and finally parturition.