A pregnant patient in her third trimester begins feeling dizzy and lightheaded while supine in the dental chair. What is the most likely cause?

In the third trimester, the enlarged gravid uterus compresses the inferior vena cava (reducing venous return to the heart) and the aorta (reducing cardiac output) when the patient lies in the supine position. This causes supine hypotensive syndrome (aortocaval compression syndrome), manifesting as dizziness, lightheadedness, nausea, and hypotension. Management: immediately tilt the patient to her left side (left lateral decubitus position), which displaces the uterus off the vena cava and restores venous return. All third-trimester dental patients should be positioned with a pillow under the right hip or tilted 15–30° to the left to prevent this complication.