Contribution of maternal circulation to blood-borne progesterone in the fetus: I. Studies on human subjects

Abstract

Tritiated progesterone was infused intravenously at a constant rate into three pregnant volunteers in labor, for at least two hours before delivery. Blood samples from a maternal peripheral vein and from umbilical vessels were taken at birth. Comparison of the specific activities of progesterone in these samples indicated that 10 per cent or less of the hormone in fetal circulation is derived from transfer of maternally circulating progesterone. After consideration of reported values of umbilical vein blood flow at term and measured arteriovenous differences in concentrations of progesterone in umbilical vessels, the secretion rate of the placental hormone toward the fetus was estimated to be about 1/10 of the rate of secretion of progesterone toward the maternal circulation. About 1 per cent of the maternally circulating hormone was found to cross the placenta.