Serial study of factors influencing changes in cardiac output during human pregnancy.

Serial hemodynamic measurements were performed in 13 women on two occasions before conception and then at monthly intervals throughout pregnancy. Cardiac output (CO) was measured by Doppler and cross-sectional echocardiography at the aortic, pulmonary, and mitral valves. Cardiac chamber size and ventricular function were investigated by M-mode echocardiography. CO increased from a mean of 4.88 l/min before the conception to a maximum of 7.21 l/min at 32 wk, the increase being significant by 5 wk after the last menstrual period. Heart rate and left ventricular performance increased during the first trimester. Heart rate increased further during the second trimester, during which left atrial and left ventricular end-diastolic dimensions increased, suggesting an increase in venous return. Derived values of total peripheral vascular resistance fell during the first 20 wk. These changes were associated with a progressive increase in valve orifice area and left ventricular wall thickness during pregnancy.