Estrone, 17 beta-estradiol, and cortisol in serum of peripartum rhesus monkeys.

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Prepartum maternal serum concentrations of progesterone and 5α-pregnane-3,20-dione are reportedly higher at 2100 h than at 0900 h (Sholl, S. A., J. A. Robinson, and R. C. Wolf, Endocrinology, in press). These findings were interpreted to mean thatplacental progestin secretion is elevated during the evening. The present study was initiated to determine whether the fetoplacental unit exhibits a similar pattern of secretory activity as might be reflected in a temporal alteration in serum estrone, 17β-estradiol, and cortisol concentrations in the mother and whether there are consistent daily changes in morningevening steroid levels preceding parturition. Steroid levels were measured by radioligand assays of maternal systemic serum obtained at 0900 and 2100 h around the time of parturition. Estrogen levels in the four animals examined increased during the 9 days immediately preceding parturition to maximum levels on the day of delivery. In direct contrast to the reported diurnal alteration in progesterone and 5a-pregnane-3,20-dione, the prepartum concentrations of estrone, 17β-estradiol, and cortisol were substantially higher at 0900 than at 2100 h. No change occurred in the magnitude of this difference as gestation advanced. After parturition, serum estrogen levels were substantially lower. In two monkeys nursing their infants, no diurnal variation in hormone concentration was detected during the 16 days after parturition; thus, it would appear that the prepartum variation originated in the fetoplacental unit and not in the ovary. Unlike estrogen, day to day cortisol levels did not change during the final 2 weeks of gestation or immediately after delivery. However, morning cortisol concentration on day 16 postpartum was substantially lower than on the day after parturition. A nocturnal decline in hormone concentration, which probably originated in the mother, was observed in both pre- and postpartum animals. The prepartum increase in estrogen levels suggests an estrogen involvement in parturition. Moreover, the diurnal fluctuation in circulating estrogen may be important in restricting parturition to the evening or early morning. The initiator(s) of this prepartum estrogen cyclicity remains to be elucidated; however, this change may be effected by an alteration in fetal adrenal activity mediated by negative feedback of cortisol on fetal pituitary ACTH secretion.