Amniotic fluid protease activity, protease inhibitory activity, and fetal lung maturity

Amniotic fluid acid protease and acid protease inhibitory activities were examined in normal pregnancies as a function of gestational age. The acid proteolytic activity of the amniotic fluid is almost constant during gestational weeks 16-29 (26 +/- 13 micrograms globin/ml/2 hrs, mean +/- SD, n = 64). The activity sharply increases after 29 weeks in a time-dependent fashion and reaches a value of 302 +/- 89 (mean +/- SD, n = 13) at 39-40 weeks gestation. Under standard conditions, the ability of amniotic fluid to inhibit bovine pepsin declined during gestation in a linear fashion from 44 +/- 13% (mean +/- SD, n = 36) at 16-18 weeks to 9 +/- 10% (mean +/- SD, n = 41) at 36-40 weeks. A correlation coefficient of r = 0.72 was found between pepsin inhibitory activity and gestational age. No consistent change was noted in the extent of inhibition of the endogenous acid protease throughout pregnancy. In 61 amniotic fluid samples, a correlation coefficient of r = 0.70 was found between acid protease activity and the lecithin/sphingomyelin (L/S) ratio. During the course of this study, five cases of respiratory distress syndrome (RDS) were diagnosed clinically. All five infants had a low protease activity (55 +/- 22 micrograms globin/ml/2 hr, mean +/- SD) as well as a low L/S ratio (0.68 +/- 0.20, mean +/- SD). In contrast, no case of RDS of the newborn was observed among 29 pregnancies with high protease activity and a high L/S ratio. The present observations may suggest a predictive value of amniotic fluid acid protease activity in assessment of fetal lung maturity.