What happens to blood pressure in a pregnant woman who is in a supine position?

In a pregnant woman who is in a supine position, blood pressure typically decreases. This phenomenon is primarily due to the pressure exerted by the enlarging uterus on the inferior vena cava, which is a large vein responsible for returning blood from the lower body to the heart. When a pregnant woman lies flat on her back, this compression can impair venous return, resulting in reduced cardiac output and subsequently lowering blood pressure.

Additionally, the hormonal changes during pregnancy can lead to vasodilation, further contributing to lower blood pressure. It's important to monitor pregnant patients closely in this position to prevent potential complications such as supine hypotensive syndrome, which can cause dizziness and reduced blood flow to the fetus.

The other options do not accurately describe the physiological changes that occur in this situation. An increase in blood pressure is not typically observed, nor is there a stable or fluctuating pattern in healthy pregnancies when lying supine; rather, a notable drop in blood pressure is the primary concern and observation.