Unconjugated and total estriol in human amniotic fluid--changes in the ratios between the two estriol levels with advancing gestational age.

In 172 amniotic fluid samples of the early second and the third trimester from 115 normal pregnancies the concentrations of unconjugated and total estriol were measured by means of radioimmunoassay. The mean concentrations of unconjugated estriol in amniotic fluid rose from 1.22 ng./ml. in the gestational weeks 15-18 to 7.82 ng./ml. in the gestational weeks 37-40, that is 6.4 times the former value. In contrast, the mean concentrations of total estriol in amniotic fluid increased more strongly, namely from 27.51 ng./ml. in the gestational weeks 15-18 to 777.15 ng./ml. in the gestational weeks 37-40, that is 28.3 times the former value. With advancing pregnancy the proportion of unconjugated estriol to total estriol in amniotic fluid fell, obeying a significant correlation, from 4.43% in the gestational weeks 15-18 to 1.01% in the gestational weeks 37-40. The presumable reasons for this are the rising conjugation rate of estriol excreted with fetal urine, concomitant to fetal maturation, and the relative transfer diminution of unconjugated estriol from maternal plasma into amniotic fluid with the advance of gestation.