Adaptations of maternal cardiovascular and renal physiology to pregnancy

Major cardiovascular and renal changes occur during pregnancy to ensure optimal development of the placenta and fetus, and to protect the health of the mother. These changes include increases in the cardiac output and reductions in the systemic vascular resistance and systemic blood pressure. A clear understanding of all these changes is necessary in order to understand how disruptions in the normal physiological responses to pregnancy result in pathological conditions associated with pregnancy such as pregnancy-induced hypertension and preeclampsia. Physiological hematological changes in pregnancy To understand the cardiovascular changes which occur during pregnancy it is important to consider first of all the hematological changes that occur, as these will have an important influence on the changes which are required in pregnancy by the cardiovascular system. Blood volume In pregnancy, plasma volume increases by over a litre from 2600 mL to approximately 5000 mL. This occurs early in pregnancy and plateaus by approximately 32 weeks' gestation (Figure 1.1). This increase in plasma volume is approximate and correlates with the size of the fetus. Multiple pregnancies therefore are associated with a greater increase in plasma volume while pregnancies where the fetus is growth restricted are associated with a suboptimal increase in plasma volume. The blood volume in pregnant women at term is approximately 100 mL/kg [1].Hematological indices The red cell mass increases in a linear fashion by approximately 30% throughout pregnancy (Figure 1.1), from a non-pregnant level of 1400 mL to approximately 1700 mL [2].