Placental clearance rate of maternal plasma androstenedione through placental estradiol formation: an indirect method of assessing uteroplacental blood flow.

The metabolism of androstenedione (A) by the placenta in late pregnancy and the early puerperium was studied. The metabolic clearance rate of A (MCR-A) was increased in pregnant women, 2,825 +/- 207 L/24 hr (mean +/- SEM), compared to 2,020 +/- 140 L/24 hr in nonpregnant women of similar body weight. The immediate puerperal MCR-A was 2,538 +/- 50 L/24 hr. Therefore, approximately 10% of maternal plasma A was cleared by the placenta. In the latter half of pregnancy, the extent of conversion of maternal plasma A through estradiol formation, (rho)AE2, was increased, whereas in the immediate puerperium it was normal, 0.018. Moreover, 90% of aromatase activity was attributed to the placenta in late pregnancy. From these data, we computed that the placental clearance rate of A through estradiol (PCAE2) from whole blood was 497 +/- 41 ml/min in women with a single fetus and 691 +/- 102 ml/min in women with twin fetuses. Thus, it appears that the PCAE2 is a sensitive index of maternal-placental perfusion.