Plasma progestagens and oestrogens in fetus and mother in late pregnancy.

Normal Thoroughbred and catheterized Pony mares and their fetuses were used. Fetal oestrogen and progestagen concentrations in late gestation were much higher than maternal values. A major feature of the umbilical steroid concentrations was a large venous-arterial difference in progestagens and total oestrogens throughout late gestation which may indicate a metabolic cycle in the fetus between progesterone and other steroid metabolites. Metabolites of 20alpha-dihydroprogesterone were present in high concentrations in maternal and fetal plasma. In Thoroughbreds, and to a lesser extent in Pony mares, plasma progestagens rose towards parturition while total plasma oestrogen fell. The only oestrogen which showed a rise in concentration near term was oestradiol-17beta. In one Pony mare and fetus, fetal plasma progestagen fell 24 to 36 hr before parturition when maternal peripheral progestagen concentrations remained high.