Changing patterns of steroid production in the fetus and placenta and their effects on development.

A variety of radioimmunoassays specifically characterized for use with fetal and maternal ovine plasma have been used to measure steroid hormone concentrations in small plasma samples drawn simultaneously from mother and fetus. By repeated sampling from the same animal preparation the changes over time of plasma concentrations of cortisol, pregnenolone and its sulphate, and oestrone and oestrone sulphate have been systematically studied. In animals delivering spontaneously at term the fetal plasma cortisol concentration rises between 120 and 130 days gestation and plays an important part in the maturation of several vital physiological systems, including the fetal thyroid axis. In normal term deliveries and premature deliveries induced by synthetic adrenocorticotropin (ACTH(1-24), 1 microgram h-1), the concentration of oestrone sulphate in maternal plasma increases before the fetal plasma concentration. Fetal and maternal oestrogens are probably important in the control of the low-grade tonic myometrial activity that occurs throughout gestation, in the initiation of labour and in the control of uterine blood flow. Low-grade tonic myometrial activity affects fetal oxygenation, fetal breathing and the fetal sleep state and may constitute a pathway through which the mother influences fetal development, with important physiological and possibly pathological consequences.