The plasma estradiol as an index of fetoplacental function

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Abstract
A radioligand assay has been developed for the measurement of unconjugated 17β-estradiol in as little as 0.01 ml of pregnancy plasma employing rabbit uterine cytosol as specific binder and activated charcoal as nonspecific absorbant. Large numbers of samples could be processed simultaneously at relatively little expense and results were obtainable within 3 hr. The procedure was sensitive to 1 ng/ml. No diurnal or positional variations were found. Values from 250 unselected normal patients showed a constantly rising mean plasma E2 from 18 to 35 wk gestation from 4.5 to 14 ng/ml. From 35 to 40 wk, mean E2 rose only to 15 ng/ml and the range of values increased substantially. When 22 normal pregnant subjects were followed serially, rising levels of plasma E2 were found with no significant fall ever seen. By contrast, patients exhibiting fetal distress generally had falling or reduced E2 levels. However, 3 cases of Rh isoimmunization had elevated or normal E2 concentrations even after clearcut evidence of fetal demise. A decrease of 45% in E2 concentration was associated with intraamniotic instillation of hypertonic saline prior to delivery supporting the view that placental conversion of maternal adrenal precursors is responsible for about half of the E2 production in pregnancy. The postpartum clearance of endogenous E2 was measured and found to fit a two compartment model with mean half-time of 22 min and 7 hr. Follicular phase levels of E2 were attained by 35 hr postpartum. The concentration of unconjugated E2 in pregnancy plasma correlated as well with the state of the placenta as other placental hormone measurements and holds promise of being a rapid, inexpensive, and reliable method of following patients with high-risk pregnancies in a variety of clinical settings.