The many physiological and hormonal changes occurring during pregnancy have the potential to affect a woman's cardiovascular responses to aerobic exercise. For example, it seems that increased mass and dilation of the left ventricle allow maintenance of wall stress, ensuring adequate pumping function throughout gestation. Also, a woman's cardiac output response to exercise is typically increased during pregnancy, owing to alterations in both heart rate and stroke volume. However, mean arterial pressure response is not usually affected by pregnancy because the increased blood flow to the uteroplacental unit, skin, and kidneys results in a concomitant decrease in systemic vascular resistance. It should be noted that there is no clear agreement on the mechanisms behind hemodynamic responses to exercise throughout gestation. Indeed, a pregnant woman's cardiac output during physical activity may be affected by (1) gestational age, (2) body position, (3) exercise intensity and modality, and (4) fitness level.