Ratio of unconjugated to total estriol in uncomplicated third-trimester pregnancy

The serum unconjugated estriol, as a percent of total estriol, has been determined in third-trimester pregnancies studied longitudinally. The unconjugated fraction in any specific sample ranged from 4.1% to 21.6% of the total. The mean percent unconjugated estriol in 34 patients sampled 7 to 15 times each between 32 and 40 weeks of gestation ranged from 5.3% to 13.6%. Higher serum total estriol concentration correlates with a lower percent of unconjugated. The mean percent unconjugated decreases during the last eight weeks of gestation from 11.4% to 8.4%, with a calculated regression line of Y = 23.323--0.368X. Thus, the concentration of serum unconjugated estriol is relatively more constant. The majority of patients demonstrate a narrow range of percent serum unconjugated estriol. These circumstances, combined with ease of assay, support the use of serum unconjugated estriol in pregnancy estriol monitoring.