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The effects of orthostatic stress on cardiac output and systemic vascular resistance have not been previously studied in pregnancy. A Doppler technique of measuring cardiac output was used to study 15 nonpregnant women, 14 pregnant women in the first trimester, and 16 pregnant women in the third trimester. Subjects were studied in recumbent, sitting, and standing positions. In each group, the change from recumbent to standing positions resulted in a fall in cardiac output of 1.7-1.8 L/minute (P = .001). Systemic vascular resistance rose in each group (P = .001). The magnitude of the rise was greatest in nonpregnant women and smallest in the third trimester (P = .005). The observed fall in cardiac output and rise in systemic vascular resistance may be clinically significant in pregnancies complicated by uteroplacental insufficiency.