The major hemodynamic changes induced by pregnancy include an increase in cardiac output, sodium and water retention leading to blood volume expansion, and reductions in systemic vascular resistance and systemic blood pressure. These changes begin early in pregnancy , reach their peak during the second trimester, and then remain relatively constant until delivery (figure 1). They contribute to optimal growth and development of the fetus and help to protect the mother from the risks of delivery, such as hemorrhage. Knowledge of these cardiovascular adaptations is required to correctly interpret hemodynamic and cardiovascular tests in the gravida, to predict the effects of pregnancy on the woman with underlying cardiac disease, and to understand how the fetus will be affected by maternal cardiac disorders.
The cardiovascular changes associated with normal pregnancy will be reviewed here. The management of specific cardiac disorders, such as acquired and congenital heart disease, heart failure, and arrhythmias, are discussed separately. (See "Acquired heart disease and pregnancy" and "Pregnancy in women with congenital heart disease: General principles" and "Management of heart failure in pregnancy".)
CHANGES IN BLOOD VOLUME
Expansion of the plasma volume and an increase in red blood cell mass begin as early as the fourth week of pregnancy, peak at 28 to 34 weeks of gestation, and then plateau until parturition [2-4]. Plasma volume expansion is accompanied by a lesser increase in red cell volume (figure 2) . As a result, there is a modest reduction in hematocrit, with peak hemodilution occurring at 24 to 26 weeks. The blood volume in pregnant women at term is about 100 mL/kg .
Plasma volume — Total body volume expansion is accompanied by retention of 900 to 1000 meq of sodium and 6 to 8 liters of water, distributed among the fetus, amniotic fluid, and extracellular and intracellular spaces [7,8]. Plasma volume increases by 10 to 15 percent at 6 to 12 weeks of gestation [9-11], expands rapidly until 30 to 34 weeks, after which there is only a modest rise. The total gain at term averages 1100 to 1600 mL and results in a plasma volume of 4700 to 5200 mL, 30 to 50 percent above that found in nonpregnant women [4,9]. Mild edema is commonly seen.
Plasma renin activity tends to be increased and atrial natriuretic peptide levels are slightly reduced, suggesting that the increase in plasma volume represents underfilling due to systemic vasodilatation and the ensuing rise in vascular capacitance, rather than true blood volume expansion which would produce the opposite hormonal profile (low plasma renin activity, elevated atrial natriuretic peptide) [12,13]. Furthermore, the degree of sodium retention is physiologically regulated, as increasing sodium intake does not produce further volume expansion . Humoral factors that contribute to volume regulation during pregnancy are discussed separately. (See "Maternal endocrine and metabolic adaptation to pregnancy", section on 'Adrenal gland'.)