During pregnancy, the cardiovascular system changes to adapt to the demands of the uteroplacental unit. CO starts to increase from the first trimester by 35-40%, to a 50% rise by the end of the second trimester. It remains at this level during the third trimester 1. The increase in CO occurs due to an increase in HR by up to 25% and SV by up to 30%. There is a further rise in CO during labour and delivery, which is transient.