Plasma progesterone levels were determined in the crab-eating macaque (Macaca fascicularis) during the menstrual cycle and pregnancy, including parturition. Menstrual cycles averaged 31 days in duration. Progesterone levels rose above 1 ng/ml plasma about 15 days prior to the onset of menstruation with peak values (7.8 ng/ml) observed on day 7 prior to menstruation. Progesterone levels declined to > 1 ng/ml 1 4 days before menstruation. Two animals studied daily for the first 30 days postbreeding, and then 2 times weekly, had 2 distinct progesterone peaks, one about IS days postbreeding and another at about 34 days with a low point in progesterone levels observed between these peaks at about days 23 to 25. The progesterone decline which followed the second peak reached minimal levels (1 to 2 ng/ml) by about day 45 of gestation. Progesterone levels increased gradually throughout the rest of gestation with average values of about 4 ng/ml observed the last 2 weeks prior to parturition. No significant changes were noted during the last 16 days of gestation. Significant progesterone levels (2.6 ng/ml) were still present by about 12 hr postdelivery with no significant decrease observed during the next 48 hr. This suggests that complete withdrawal of progesterone is not a necessary prerequisite for delivery and also that corpus luteum activity may continue in the postpartum period.
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