Estriol in pregnancy. IV. Normal concentrations, diurnal and/or episodic variations, and day-to-day changes of unconjugated and total estriol in late pregnancy plasma.

Normal values of unconjugated, total, and immunoreactive (measured without extraction) plasma estriol (E3) have been determined from radioimmunoassay data obtained in 217 uncomplicated third-trimester pregnancies. Small but significant diurnal variations in unconjugated and total plasma E3 have been observed in a study comprising 12 women who were hospitalized during late pregnancy for diabetes, toxemia, or placenta previa. The late morning decreases in unconjugated and the afternoon/evening decreases in total plasma E3 concentrations, averaging some 10 to 15 per cent, were overshadowed by considerable episodic fluctuations and may thus be clinically irrelevant. Day-to day changes of unconjugated and total plasma E3 concentrations in late pregnancy were similar and smaller than changes in urinary E3 measurements. The urinary E3/creatinine ratio, however, reducing inadequacies of 24 hour urine collections, varied less than unconjugated or total plasma E3. The data suggest that a decrease in unconjugated or total plasma E3 must exceed 40 to 45 per cent of the mean of the three preceeding determinations if it is to be considered a signal of fetal distress.