SummaryIt is well known that corticoids act to induce fetal lung maturation when administered to the fetus or mother. However, their physiologic role has been recently questioned with regard to stimulation of surfactant production.We investigated the process of fetal lung maturation to determine whether or not it is associated with changes in amniotic fluid cortisol.Sixty-two amniotic fluid samples from 53 patients were obtained by transabdominal amniocentesis strictly due to maternal and/or fetal clinical indication, according to the course of pregnancy and the maternal and/or fetal status.The L/S ratio was measured on thin-layer plates by reflectance densitometry and calculated as previously described by Glucket al.Cortisol levels in amniotic fluid were measured by radioimmunoassay using commercial reagent kits (Gamma coat I125cortisol; Clinical Assays, Inc.).The level of amniotic fluid cortisol increased with gestational age from wk 30 to 41; a sharp increment was observed at 39 to 41 wk.No significant differences were found when the mean values of cortisol were compared at wk 30 to 32, 33 to 35, and 36 to 38. When the fourth group was included (39 to 41 wk), a statistically significant difference was found (F= 4.63;P< 0.01).There was a progressive increase in L/S ratio during gestation as has already been described, plus a value greater than 2 was found during the 33 to 35 wk period. The statistical analysis performed on L/S ratios at weekly intervals 30 to 32, 33 to 35, and 36 to 38 showed a significant difference in the mean values in the groups studied (F= 6.84;P< 0.005).Therefore, the increment of L/S ratio over 2 was already observed during the 33 to 35 wk period of gestation, whereas the mean cortisol amniotic fluid values remained unchanged between the 30th and the 38th wk. The cortisol peak was observed only during the 39 to 41 wk period.