The cardiovascular system undergoes remarkable adaptive changes in pregnancy. Through a complex network of interrelated pathways the physiological response to the conceptus ensures an adequate blood supply and oxygen gradient to meet the demands of the feto-placental unit whilst maintaining normal maternal blood pressure. This is achievable because the increase in maternal cardiac output and blood volume which characterise normal pregnancy occur with an associated and profound decrease in peripheral vascular resistance. Longitudinal studies using Doppler ultrasound and echocardiography indicate that the decline in peripheral vascular resistance occurs very early and data from the pregnant baboon and pregnant women suggest that this is the stimulus which activates the renin-angiotensin aldosterone axis and the resultant rise in plasma volume.