What causes supine hypotension late in pregnancy?

Supine hypotension late in pregnancy occurs primarily due to the partial occlusion of the vena cava and aorta when a pregnant individual lies flat on their back. As the uterus grows larger, it can compress the inferior vena cava, which is responsible for returning blood to the heart from the lower body. This compression leads to a reduction in venous return, resulting in decreased cardiac output and subsequent hypotension.

Additionally, the compression can also affect the aorta, which can further compromise blood flow. This situation is particularly significant during the third trimester, when the uterus is at its largest and puts more pressure on these major blood vessels. Symptoms can include dizziness, palpitations, or even fainting when the person is in a supine position.

The other options address different mechanisms or factors that are not the primary cause of supine hypotension. For example, decreased peripheral collateral circulation, while an important concept, does not directly relate to the acute condition of hypotension in the supine position. Increased blood flow to the placenta does not cause hypotension but rather represents a compensatory mechanism as the body adapts to increased demands during pregnancy. Uterine contractions during labor may impact maternal hemodynamics differently but are not directly connected to the phenomenon of