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

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Abstract
Progesterone and cortisol were measured in the peripheral blood of pregnant women using radioimmunoassay. Ten women were between 18 and 25 wk of pregnancy and 10 women were between 32 and 38 wk of pregnancy. Blood for the hormone determinations was drawn at hourly intervals. A significant circadian progesterone rhythm was found with the nadir at 0800 h and the peak around midnight. The progeserone rhythm showed a significant inverse relation to the cortisol rhythm (.tau. = -0.310). No difference was detected between the 2 groups for progesterone rhythm or its inverse relation to the cortisol rhythm.