| Abstract: | Catheters were placed in maternal and fetal vessels in four ewes in the last month of gestation. Tritiated progesterone (P) was infused at a constant rate into a maternal peripheral vein. Specific activities and endogenous concentrations of P were measured in blood samples drawn from a maternal artery and either a fetal femoral artery or an umbilical vein. Comparison of the specific activities of P in fetal and maternal samples revealed that direct secretion of the placental hormone into the umbilical vein accounted for 90 to 93 per cent of the hormone in the fetal circulation. Estimates of the rates of production of P in the fetal circulation, obtained by multiplying metabolic clearance rates previously reported and concentrations of P in a fetal femoral artery, indicate that placental P is preferentially secreted toward the maternal circulation. |
