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 2,539 ± 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, (ρ)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.