There is an increase in cardiac output during pregnancy from about 4.5 to 6.7 litres, with a further rise during the first and second stages of labour. Despite this there is not, in a normal pregnancy, an increase in blood pressure due to an accompanying decrease in peripheral resistance.
The increased cardiac output is achieved by both an increase in heart rate (averaging 15 beats/min) and an increase in stroke volume.
It is not unusual for women to complain of faintness when lying supine during late pregnancy. This is caused by vena caval compression by the enlarging uterus and poor paravertebral collateral circulation, which in turn reduces venous return and cardiac output to cause a reduction in blood pressure.
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