**Core Concept**
Orthostatic hypotension is a condition characterized by a sudden drop in blood pressure upon standing or changing positions, leading to symptoms such as faintness or dizziness. In pregnancy, this can be exacerbated by the physiological changes that occur during full-term gestation.

**Why the Correct Answer is Right**
During the third trimester of pregnancy, the expanding uterus compresses the inferior vena cava, reducing venous return to the heart. This results in decreased cardiac output and a subsequent drop in blood pressure when the patient stands or assumes an upright position. The body compensates for this decrease in blood pressure by releasing vasopressors such as adrenaline, but this is often insufficient to prevent symptoms of orthostatic hypotension. Turning to a side or sitting position alleviates these symptoms by reducing the compression of the inferior vena cava and improving venous return to the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the specific physiological changes that occur during pregnancy.
**Option B:** This option is incorrect because it does not take into account the role of the expanding uterus in compressing the inferior vena cava.

**Clinical Pearl / High-Yield Fact**
The "supine hypotension syndrome" is a classic complication of pregnancy, where women experience symptoms of orthostatic hypotension when lying on their backs due to compression of the inferior vena cava. This can be alleviated by turning to a side or using a wedge to elevate the hips.

**Correct Answer: C. Supine hypotension syndrome.**