Variability in unconjugated and total estriol in serum during normal third trimester pregnancy

We determined unconjugated and total estriol concentrations in serum during the third trimester in 34 normal gestations. Data were obtained weekly for 13 women for as long as nine weeks; 21 others were studied daily for up to 15 days. No correlation between birth weight and either estriol fraction was demonstrated. Between-patient variability was less for unconjugated estriol and was similar for weekly and daily data. Within-patient variability was also less for unconjugated estriol and, as expected, was less for daily than for weekly data. The CV for daily samples, within patient, averaged 13.0% for unconjugated estriol and 20.3% for total estriol. Our data support the daily determination of unconjugated estriol in serum, evaluated for percent changes from single or mean preceding values, as the preferred method of monitoring estriol during pregnancy.