Circadian rhythm of serum progesterone levels in human pregnancy and its relation to the rhythm of cortisol.

Progesterone and cortisol were measured in the peripheral blood of pregnant women using radioimmunoassay. Ten women were between 18 and 25 weeks of pregnancy and 10 were between 32 and 38 weeks of pregnancy. Blood for the hormone determinations was drawn at hourly intervals. A significant circadian progesterone rhythm was found with the nadir at 8.00 h and the peak around midnight. The progesterone rhythm showed a significant inverse relation to the cortisol rhythm (tau = 0.310). No difference was detected between the two groups for progesterone rhythm or its inverse relation to the cortisol rhythm.