Maternal blood volume and cardiac output during pregnancy: a hypothesis of endocrinologic control.

During the course of gestation the increase of maternal total blood volume and cardiac output may result from two mechanisms acting in concert: 1) the production of several hormones by the fetus and the placenta, and 2) the uteroplacental circulation acting as an arteriovenous shunt. Plasma volume appears to increase as a consequence of renal Na+ reabsorption and water retention, which result from increased aldosterone production via the renin-angiotensin system, as a consequence of placental estrogen production. This estrogen production in turn results from increasing availability of the estrogen substrate dehydroepiandrosterone (chiefly from the fetal adrenal gland). Erythrocyte volume increases as a consequence of the erythropoietic effect of placental chorionic somatomammotropin, progesterone, and perhaps prolactin. In addition, maternal blood volume increases in response to the uteroplacental circulation functioning as a low-resistance circuit. In turn, this increases cardiac output and nutrient delivery for further growth of the products of gestation. Thus there may exist a feedback mechanism whereby fetal growth and, in particular, increasing steroidogenesis by the developing fetal adrenal gland result in the maternal cardiovascular adaptations to pregnancy that optimize further fetal development. In certain complications of pregnancy the less-than-normal maternal blood volume increase may result from failure of these mechanisms, while in turn contributing to the further genesis of these disorders.