The weight of the gravid uterus can have dramatic effects on the cardiovascular system. The reasons for this are illustrated; in the unmodified supine position, the uterus can compress the great vessels against the lumbar vertebral bodies. The pressure in the aorta is higher than that in the inferior vena cava (IVC), so aortic blood flow is usually affected to a lesser extent than that in the IVC.

IVC compression and obstruction causes a fall in venous return to the heart, and can result in marked falls in maternal cardiac output and blood pressure.