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The regulation of corticosteroids during late pregnancy and their role in parturition.

Some of the mechansims by which fetal adrenal growth and steroidogenesis are stimulated during late pregnancy are discussed and some of the functions of the increase in cortisol production particularly in relation to the initiation of parturition and to the maturation of organ systems necessary for extrauterine life are considered. In sheep there is a gradual rise in the concentration of cortisol in fetal plasma beginning 10-15 days prepartum with a more rapid increase during the last 2 or 3 days of gestation. This increase in cortisol is abolished by fetal hypophysectomy. The fetal adrenal can be stimulated to secrete cortisol by the continuous infusion of ACTH into the fetus for 2-5 days; this leads to premature parturition. In humans a decrease in the concentration of ACTH in cord plasma during late pregnancy has been reported. A good correlation has also been demonstrated between the concentration of prolactin in fetal plasma and growth of the fetal adrenal glands. In sheep the increase in fetal cortisol production during the last 7-10 days of pregnancy is closely followed by a decrease in progesterone during the last 1-4 days and by an increase in unconjugated estrogen during the last 24 hours of gestation. The increase is estrogen is associated with an increase in prostaglandin production and with the onset of active labor. In primates there is a greater degree of permeability to cortisol than in domestic animals and there is no evidence for placental enzyme induction or activation. Cytosol and nuclear binding sites for glucocorticoids appear in fetal lung tissue before the appearance of surfactant in alveolar speces. In human fetal lung glucocorticoid receptors have been demonstrated by 9 weeks of gestation. Glucocorticoids are known to be involved in the maturation and biochemical differentiation of other tissues in the fetus. Many of the changes attributed to cortisol will proceed although at a slower rate in the absence of a prepartum glucocorticoid surge. Cortisol may function as a tuner that synchronizes and coordinates those maturational processes needed for extrauterine life.