Relation between diurnal changes in peripheral plasma progesterone, cortisol, and estriol in normal women at 30-31, 34-35, and 38-39 weeks of gestation.

To determine the relation of diurnal changes in plasma progesterone to those in cortisol and estriol we measured the concentrations of progesterone, cortisol and estriol in samples of plasma taken at 30- to 60-min intervals throughout 24 h from women at 30-31, 34-35, and 38-39 weeks of gestation. Plasma progesterone showed a significant diurnal rhythm at 30-31 and at 34-35 weeks of pregnancy, with troughs at 04.30-10.00 h. Major peaks occurred between 15.30 and 02.30 h. There was no diurnal rhythm in progesterone at 38-39 weeks. Plasma progesterone showed a significant negative correlation with plasma cortisol at 30-31 and 34-35 but not at 38-39 weeks. Plasma progesterone showed a significant positive correlation with estriol at 34-35 and at 38-39 weeks. We suggest that daily fluctuations in plasma progesterone may be related to the concentration of plasma cortisol, either directly by competition for binding sites on transcortin, or indirectly after modulation of fetal pituitary-adrenal function by maternally derived glucocorticoid.