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Biology of Reproduction, Vol 12, 335-345, Copyright © 1975 by Society for the Study of Reproduction
1 Department of Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261 Circulating levels of progesterone and estrogen in the pregnant rhesus monkey are indistinguishable
from those of the nonpregnant animal until 12 days after the LH peak, when the first significant rise in
chorionic gonadotropin (RhCG) levels is noted concomitant with an abrupt elevation in progesterone
levels (the corpus luteum rescue). Serum RhCG concentration rises rapidly (doubling time = 2 days) to
a peak at about day 23 of pregnancy, then falls to undetectable levels at day 35, and remains
undetectable throughout the rest of gestation. Serum estrone and estradiol levels rise and fall with
RhCG titers in early pregnancy, reaching a nadir on day 35 of gestation before placental estrogen
secretion (largely estradiol) supervenes. Circulating estrogens reach a high and variable plateau at about
day 80 of pregnancy and rise to maxima on the day prior to parturition. Serum progesterone levels in
early pregnancy, while rising abruptly at the outset of RhCG secretion, fall in the face of waxing RhCG
titers and increase again only as RhCG levels fall. Ovariectomy on day 23 of pregnancy causes a sharp
fall in circulating levels of estrogens and no compromise of the continuation of pregnancy. These data suggest that the order of hormone production by the placenta is RhCG. first discernable
at about 10-11 days after ovulation, followed by progesterone, whose secretion by the placenta is
initiated prior to day 23 of pregnancy since ovariectomy at this time causes no fall in circulating
progesterone levels, and lastly, estradiol and estrone, which rise rapidly in peripheral serum after day 35
of pregnancy. These data support the hypothesis that the shortlived stimulation of progesterone secretion by the
corpus luteum in early pregnancy is caused by the initiation of placental RhCG secretion and that the
failure of luteal progesterone secretion to be maintained in the face of rising RhCG titers may be an
expression of the luteolytic effect of the increased luteal estrogen production also stimulated by this
gonadotropin.
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