A pregnant patient reports feeling light-headed when reclined in the dental chair. What is the most likely cause?

In later pregnancy, the gravid uterus can compress the inferior vena cava when the patient lies supine, reducing venous return to the heart and causing hypotension and light-headedness — this is supine hypotensive syndrome of pregnancy. It's managed by positioning the patient on her LEFT side (or otherwise tilted) to relieve compression. Orthostatic hypotension relates to position changes (lying to standing), vasovagal syncope is typically triggered by fear/pain/stress, and vertigo is a balance/inner-ear symptom — none specifically explain reclining-related symptoms in a pregnant patient as well as supine hypotensive syndrome.