Placental secretion of unconjugated estrone, estradiol and estriol into the maternal and the fetal circulation.

Abstract

The plasma concentrations of unconjugated estrone (E1), estradiol (E2) and estriol (E3) were measured by radioimmunoassay in plasma samples of umbilical artery (U.A.), umbilical vein (U.V.), retroplacental space (R.P.S.) and maternal peripheral vein (M.P.V.) of 5 patients at 10–20 weeks of pregnancy and 5 patients at term. The plasma concentrations of the 3 classical unconjugated estrogens were 4–7- fold higher in R.P.S. than in M.P.V. and 2-fold higher in U.V. than in U.A. (P < 0.05), indicating placental secretion of these estrogens into the maternal and fetal circulation. On the basis of the differences between U.V. and U.A. plasma concentrations and reported umbilical blood flow, it was estimated that the secretion rate of unconjugated Ea into the fetal circulation of 2.7 ± 1.7 (SK) mg/day was 1–8 times higher than that of E1, and E2 combined (1.0 ± 0.5 mg/day). Secretion into the fetal circulation of unconjugated E1, E2 and E3, was estimated, however, to be about 10-fold lower than that into the maternal circulation. The fraction of the 3 unconjugated estrogens which is unbound to the plasma proteins was estimated by equilibrium dialysis at 37 C. The apparent fraction of unbound E2 in M.P.V. plasma at term of 0.65 ± 0.09% (SE) was significantly (p < 0.05) lower than that of 2.0 ± 0.18% found in the nonpregnancy state. The apparent fraction of unbound E2 in umbilical artery and vein plasma of 4.5 ± 0.4% was 7 times higher than that found in M.P.V. at term (p < 0.05). This suggests that the concentration of the sex binding protein in umbilical vein and artery plasma is lower than that in M.P.V. plasma. The mean plasma concentration of the apparent unbound unconjugated E1; E2 and E3 in U.V. which was respectively 2.5-, 2.8- and 6-fold greater than that found in M.P.V. at term, indicates that the fetus at term is exposed to high plasma concentrations of these unbound unconjugated estrogens.