Peripheral Plasma Progesterone Levels in the Ewe During Pregnancy and Parturition

Progesterone levels in peripheral plasma were similar in ewes with single and twin fetuses during the first SO days of gestation (2–3 ng/ml). A gradual upward trend in progesterone concentration began at about days 55 to 60 (week 8) in ewes carrying twin lambs and at approximately day 75 (week 11) in ewes with single lambs. Progesterone levels increased in ewes with single fetuses from about 3 ng/ml on day 75 to 9.5 ng/ml at day 125 (week 18), and in ewes with twins from 3 ng/ml on day 55 to 15.5 ng/ml at day 130 (week 19). The slopes of the regression lines bsingle and bdoubles) between days 60 and 130 were significantly different (P < 0.01). The wide variation within and between ewes, however, reduced the probability of accurately differentiating between single and twin pregnancies. The decline in progesterone concentration which began about days 130–135 (week 19) and continued to the day of parturition (9.5 ng to 4.3 ng in singles and 15.5 ng to 3.8 ng in twin pregnancies) suggests endocrine preparation for delivery begins about 2 weeks prior to parturition. Progesterone levels were about 2 ng/ml at the time of delivery of the fetus. A final reduction to 0.8 ng/ml occurred 30 min to 60 min postpartum. Prolonged gestation occurred in one ewe following bilateral surgical blockage of both fetal carotid arteries. Progesterone levels remained high during the post-term period (days 147–163). The lamb was delivered alive on day 163 by caesarean section. Most of the brain (including the pituitary and hypothalamus) was destroyed by liquefaction necrosis. This emphasizes the importance of an intact fetus for the normal induction of labor. (Endocrinology90: 144, 197