Final answer:
A pregnant woman's blood pressure may drop when she lies supine due to the inferior vena cava compression by the growing uterus, reducing venous return and, consequently, blood pressure. This is compounded by the increased blood volume and circulation needs during pregnancy.
Step-by-step explanation:
When a pregnant woman lies supine or in the right lateral decubitus position, her growing uterus can compress the inferior vena cava (IVC), a phenomenon known as the supine hypotensive syndrome or aortocaval compression syndrome. This compression reduces the return of blood to the heart (venous return), leading to a drop in blood pressure. The drop in blood pressure may be worsened by the fact that pregnant women have an increased blood volume by around 30 percent to support the fetus, which adds to the circulatory system's load, making the adjustments for blood pressure more challenging.
Furthermore, the physiological changes aimed to support the fetus, such as increased cardiac output and moderate rise in pulse, can become inadequate when the large vessels are compressed. In contrast, when a person stands up from a supine or seated position, they might experience orthostatic hypotension, which occurs because the cardiovascular system has to overcome the effects of gravity to maintain blood flow to the brain, sometimes resulting in dizziness or a brief loss of balance.