Changes in active and inactive renin throughout normal pregnancy.

Active and inactive renin were measured sequentially in 16 women throughout pregnancy and again post-partum. By 12 weeks, inactive renin was elevated 14-fold and fell slightly thereafter. By 12 weeks, active renin was 3.5-fold elevated. It continued to rise in 8 patients (Group A) until term, it remained stable in 6 (Group B) and in 2 it was quite variable. Between 12 and 32 weeks PRA and plasma aldosterone increased in Group A from 7.0 to 17.1 ng/ml/hr, and 20.0 to 95.7 ng/dl respectively and both were unchanged in Group B (6.5 to 8.1 ng/ml/hr and 26.5 to 24.6 ng/dl respectively). Inactive renin was higher in Group B than in Group A at 12 weeks (224 v 126 ng/ml/hr) but fell thereafter to the same stable but elevated level. Group A were younger (24 +/- 1.8 S.E. v. 30 +/- 2.2 years). These studies demonstrate that active and inactive renin increase early in pregnancy and remain elevated thereafter. In certain younger subjects, further increases in active renin and aldosterone sometimes occur and appear necessary for the maintenance of normal blood pressure.