Diurnal changes in the concentration of progesterone in the plasma of women at 34-35 weeks of gestation.

The concentration of progesterone has been measured in ten women at 34-35 weeks of gestation in blood samples taken at 30 or 60 min intervals during a 24 h study period. Plasma progesterone concentrations showed a distinct diurnal rhythm. Most values determined between 04.00 and 11.00 h were significantly lower than the peak concentrations measured between 13.00 and 01.00 h. The relation of these changes in plasma progesterone to those in plasma cortisol and oestrogens is discussed.