Progesterone levels in the bonnet monkey (Macaca radiata) during the menstrual cycle and pregnancy

Progesterone (P) levels in the peripheral blood of parous bonnet monkeys (Macaca radiata) were studied by a competitive protein-binding radioassay method during the menstrual cycle and in pregnancy. During the menstrual cycle P levels averaged less than .5 ng/ml during the first 9 days and then began to rise considerably between Days 9-11 reaching maximal levels on Day 18. P levels then showed a considerable decline by Day 24 of the cycle when compared with levels during Days 15-23. 2-3 days prior to menstruation P levels were less than 1 ng/ml. P levels during the first 12 days postbreeding were higher than during the luteal phase but presented a similar pattern. A definite divergence of P levels in pregnancy compared with nonpregnant animals occurred at Day 15 of pregnancy or Day 24 of the menstrual cycle when levels in pregnant animals were maintained and levels in nonpregnant animals significantly declined. This devergence suggests that there is a well-defined period in which the corpus luteum regresses if the animal is not pregnant. P concentrations began to decline on Day 35 postbreeding and continued until Days 45-50. Until Days 145-150 P levels varied and thereafter began to increase. The greatest increase in P levels was observed during the last 6 days before delivery. P levels which averaged 20 ng/ml about 12 hours before delivery declined to 2 ng/ml at 12 hours postpartum.