Total cortisol in amniotic fluid and fetal lung maturation.

Total cortisol was determined by radioimmunoassay in 48 samples of amniotic fluid obtained at various stages of normal pregnancy. Before the 34th week, all cortisol levels were less than 40 ng per milliliter. At 35 to 40 weeks the mean value was 2.4 times higher than that at 20 to 34 weeks. In pregnancies greater than 40 weeks, a further increase in total amniotic-fluid cortisol was observed, and values greater than 120 ng per milliliter were found only in patients of this group. A good rank correlation between cortisol and lecithin/sphingomyelin ratio was found in 43 samples (r = 0.83, p less than 0.001). No respiratory-distress syndrome of the newborn occurred when total amniotic-fluid cortisol was greater than 60 ng per milliliter (16 patients). Total amniotic-fluid cortisol may reflect initiation of fetal lung maturation, and may help identify pregnancies with a gestation period of over 40 weeks.