Protease inhibitors in serum and amniotic fluid during pregnancy.

Antitryptic and antichymotryptic activities in amniotic fluid and maternal serum were measured at various stages of pregnancy using the caseinolytic assay method for proteases. The inhibitory activities increased up to 20 weeks in amniotic fluid and remained in the same range up to 30 weeks; a sharp fall was observed at term. In serum, there was a gradual rise of activities with peak values around 30 weeks. The decrease with further advance of pregnancy was not as sharp as in amniotic fluid. The ratios of antitryptic to antichymotryptic activities remained fairly constant (1.54 +/- 0.07) in amniotic fluid throughout pregnancy. In maternal serum the ratio varied over a range of 1.28 to 2.67 and the increase in antichymotryptic activity was relatively greater between 16 and 30 weeks resulting in a lower ratio between the two activities. Amniotic fluid was found to contain a higher proportion of a heat stable inhibitor compared to serum and its contribution to total antitryptic activity varied from 8.7 to 15.6 per cent.