The effect of normal labor on the renin-angiotensin system in mother and fetus.

Plasma renin activity (PRA) and angiotensin I concentration (AI) in mother, umbilical vessels, and amniotic fluid were measured during 11 elective cesarean sections and after 12 normal labors. During cesarean section, PRA and AI were significantly lower in umbilical vessels than in mother or in amniotic fluid. This finding reflects the steady state in the late pregnancy. PRA and AI were significantly higher in the umbilical vessels after normal labor in comparison to the levels of cesarean section. In the mother there was no difference in PRA after normal labor and during cesarean section, but AI was significantly higher after normal labor. After normal labor no difference of PRA between mother, umbilical vessels, and amniotic fluid was found. The rise of fetal PRA and AI as a reaction to uterine contractions may be of great importance in ensuring the fetoplacental circulation during the labor.