Changes in the Levels of Progesterone, Corticosteroids, Estrone, Estradiol-17 β, Luteinizing Hormone, and Prolactin in the Peripheral Plasma of the Ewe During Late Pregnancy and at Parturition

Abstract

Serial measurements (up to four times per day) of maternal plasma progesterone, corticosteroids, estrone, estradiol-17β, LH, and prolactin have been made in six sheep, during late pregnancy and at parturition (Day 0). The mean progesterone concentration on Day –10 was 15.5 ng/ml. From Day –4 the level began to decrease and by Day 0, the mean progesterone level was 1.0 ng/ml. The mean concentrations of estrone and estradiol-17β on Day –3 (62.4 and 24.0 pg/ml, respectively) began to rise during the 48 hr preceding parturition, reaching peak values of 228.0 and 141.8 pg/ml, respectively. The mean corticosteroid level (17.3 ng/ml) rose during the 18 hr preceding parturition, reaching a peak of 38.6 ng/ml. Estrone, estradiol-17β, and corticosteroids returned to low levels within 12 hr after parturition. LH levels remained relatively constant (2.0–6.0 ng/ml) from –36 h to +30 h but prolactin levels rose sharply from about Day –2. Possible explanations for the absence of an increase in LH secretion, in response to the preparturient rise in estrogens, are discussed.