The clearance rate of maternal plasma prehormones of placental estrogen formation.

A method has been established for the determination of the placental clearance of maternal plasma prehormones by placental estrogen formation during human pregnancy. Utilizing this concept, the placental clearance is calculated as the product of the metabolic clearance rate from maternal plasma of the prehormone and its fractional conversion to estrogen by the placenta. This value equals the volume of maternal plasma cleared of that prehormone by the irreversible formation of its estrogenic product hormone per unit of time. The placental clearance rate technique can be performed by intravenous infusion of isotope-labeled prehormone and product, and the metabolic clearance rate as well as the transfer constant can be measured by techniques employing plasma values or by a combination of plasma and urinary methods. Utilizing this type of study, the placental clearance of plasma prehormones has been observed to be increased initially in women destined to develop pregnancy-induced hypertension but declines prior to the onset of hypertension and continues to decline until delivery is effected. Moreover, the administration of diuretics and/or the institution oental clearance of plasma prehormones from the maternal compartment in normal gravidas, in those destined to develop pregnancy-induced hypertension, and in gravidas who are hypertensive from any cause.