Patterns of Circulating Progesterone Concentrations During the Fertile Menstrual Cycle and the Remainder of Gestation in the Rhesus Monkey

During nonfertile menstrual cycles peripheral plasma progesterone concentrations (competitive protein binding analysis) begin to rise 2 days prior to ovulation, reach levels of 3–4 ng/ml on about the 15th day of the cycle and begin to decline 7 days later. During fertile cycles this decline is abruptly halted and reversed by striking increases in progesterone levels which exceed those observed in the luteal phase of the nonfertile cycle by 100%. The duration of this secondary progesterone peak, however, isonly 10 days, at which time progesterone concentrations return to below luteal phase levels. A third progesterone peak, 15 days in duration, then appears which terminates in low values of 1 ng/ml or less. Progesterone concentrations then rise for the fourth time, to 2–4 ng/ml, where they are irregularly maintained for the remainder of the 168-day gestation period. Progesterone falls abruptly at parturition to very low or immeasurable levels. (Endocrinology84: 45, 1969)