Effect of Postural Change on Plasma Renin and Aldosterone Concentrations in Third-Trimester Pregnancy

Abstract

Plasma renin activity and aldosterone concentration were determined radio immunologically after 30 minutes in the left lateral position and after an additional 30 minutes of supine recumbency in 26 normotensive nulliparous women between 28 and 32 weeks' gestation. Plasma renin activity decreased slightly but significantly in the supine position (P <.05). A distinct increase of plasma aldosterone was observed in 14 of the 26 subjects. The following speculation has been drawn from these results: The fetal reninangiotensin system is activated by decreased uteroplacental perfusion in the supine position, possibly followed by a diminution of blood flow to the fetal kidney, perhaps via hypoxia. Subsequently, aldosterone may also be secreted by the fetus at an increased rate. In contrast to the high molecular weight renin, aldosterone passes the placental barrier and appears in the maternal circulation.