Unconjugated cortisol in human amniotic fluid: relationship to lecithin/sphingomyelin ratio.

A specific and thoroughly validated competitive protein binding radioassay has been developed for the measurement of unconjugated cortisol in human amniotic fluid. Cortisol levels rose throughout normal gestation from 8.6 +/- 0.8 ng/ml at less than 20 weeks to 19.8 +/- 1.5 ng/ml at 30-40 weeks. Levels were significantly higher in amniotic fluid samples having mature lecithin/sphingomyelin (L/S) ratios (greater than 2), and good correlation with individual cortisol values was obtained (r = 0.79; P less than 0.001). These results support the hypothesis that cortisol plays a role in fetal lung maturation.