Interrelationship of Estrogen Concentrations in the Maternal Circulation, Fetal Circulation and Maternal Urine in Late Pregnancy

Abstract

The levels of estrone, estradiol-17β and estriol in plasma obtained from umbilical artery, umbilical vein, maternal brachial artery (or peripheral vein) and uterine vein have been estimated in a series of subjects at cesarean section and at normal delivery. Levels of estrone and estradiol-17β were higher in the umbilical vein than in the umbilical artery, presumably reflecting secretion of these hormones by the placenta and subsequent metabolism by the fetus. Comparison of estrogen concentrations in maternal and fetal circulations showed that concentrations of estrone and estradiol-17β were 2–4 times higher in the maternal than in the fetal vessels, whereas the concentration of estriol in the fetal vessels was 9–12 times higher than in the maternal. These concentration differences were shown to exist generally across the placenta. They do not necessarily reflect concentration gradients. Concentrations of estradiol-17β and estriol are significantly higher in the uterine vein than in maternal brachial artery (or peripheral vein), indicating a net release of these hormones by the pregnant uterus. No concentration differences were observed in the case of estrone. The ratio of release rates of estradiol-17β and estriol from the uterus was calculated and was compared with the observed ratios of estrone, estradiol-17β and estriol in the urine. The observed ratio of urinary estrone: estradiol-17β correspondedclosely to that expected, but the observed ratio of estriol:estradiol-17β was 2.6 times higher than that expected. This finding indicates that over 50% of urinary estriol comes from sources other than the estradiol-17β+estriol released by the pregnant uterus. It would not support the view that the additional estriol found in the urine in late pregnancy can be fully accounted for by placental estriol secretion. Five different extraction procedures failed to indicate any association of endogenous estrogens with red cells obtained from maternal peripheral blood. It is concluded, therefore, that the red cells play no significant part in the transport of estrogens late in human pregnancy.