Hormones in human pregnancy. IV. Plasma progesterone.

The plasma concentration of progesterone (P) has been measured by radioimmunoassay in maternal peripheral vein (M.P.V.) at early pregnancy and in M.P.V. umbilical artery (U.A.) and umbilical vein (U.V.) at term pregnancy. In early preganacy marked hour-to-hour fluctuation of plasma progesterone was noted. At term pregnancy plasma P levels of U.V. were higher than those of U.A. and the umbilical venous arterial differences of plasma P did not differ between male and femal fetuses. Administration of hydrocortisone and ACTH to patients scheduled to undergo cesarean section had no effect on M.P.V., U.A., and U.V. plasma P concentration. On the basis of the differences between U.V. and U.A. plasma P concentrations and reported umbilical flow it was estimated that the secretion rate of P into the fetal circulation is approximately 23 mg. per 24 hr. and would amount to approximately 10 per cent of the reported total daily production rate of P at term pregnancy. The fraction of P which is unbound to the plasma proteins was estimeated by equilibrium dialysis at 37 degrees C. The per cent unbound P in M.P.V. plasma of pregnant patients at term was not different from that of nonpregnant patients but was 40 per cent lower than that in umbilical cord plasma (P LESS THAN 0.01), and the ratio between the concentrations of unbound P and estradiol in M.P.V. increased as pregnancy progressed. Plasma P in re-eclamptic patients who subsequently sustained intrauterine fetal death had no value in assessing placental function.