Relationships between maternal hormones and weight of newborn sheep

Intact and ovariectomized mixed-breed sheep were studied in chronic experiments during the last weeks of pregnancy. Measurements were made of the rate of uterine blood flow (ml/min and ml X min-1 X kg-1, combined weight of fetus, uterus, and placenta) and of arterial and uterine venous concentrations of progestagens, estrone, and estradiol. The birth weight of the lamb was directly related to mean maternal concentrations of estrone and estradiol and to the rate of progestagen release by the uterus and its contents (ng X ml-1 X kg-1), calculated by taking the product of the rate of blood flow and the arteriovenous progestagen difference. Ovariectomy altered the relationship between birth weight and progestagen release. Data suggest that rates of progestagen release both by the fetoplacental unit and by the ovary are proportional to the birth weight of the lamb.