Timing of the luteal-placental shift in the baboon (Papio cynocephalus).

Normally cycling female baboons (Papio cynocephalus) were mated with males of proven fertility during the periovulatory period. After pregnancy was confirmed, the corpus luteum-bearing ovary was removed (CL-OVX) at day 18 (n = 1), day 20 (n = 4), day 25 (n = 3), or day 30 (n = 4) of gestation. Upon CL-OVX at day 18 or 20, there was an immediate decline in plasma progesterone to basal levels, and pregnancy was not maintained. After CL-OVX on days 25 or 30, there was a transient, small decline in plasma progesterone levels which recovered rapidly to levels found in pregnant controls, and pregnancy was maintained. Only one baboon failed to maintain pregnancy after CL-OVX on day 30, but progesterone levels remained elevated for 3 days before the termination of pregnancy suggesting that causes other than CL-OVX were responsible. In another group of three baboons, depot medroxyprogesterone acetate was administered before CL-OVX at day 20 of gestation and again at days 23 and 26. Progesterone declined sharply after CL-OVX to near basal levels but increased between days 22 and 24, and pregnancy was maintained in all cases. Estradiol levels did not decline as sharply as those of progesterone in any group, which indicates an exclusively luteal source for progesterone while estradiol comes from both the luteal and nonluteal ovaries. There is a placental increase in estradiol production which occurs much later than does the production of progesterone. The luteal-placental shift occurs between day 20 and 25 of gestation in the baboon, earlier than in women but later than in rhesus monkeys.