Vasodilatory effect of estradiol on the arterial system: This is the most likely underlying mechanism for the patient's symptoms. During pregnancy, estradiol levels increase, causing vasodilation and a decrease in peripheral vascular resistance. This can lead to dizziness and lightheadedness, especially when changing positions from supine to standing.

Cerebral blood flow insufficiency secondary to physiologic anemia of pregnancy: Although the patient's hemoglobin level is within the normal range, physiologic anemia of pregnancy can still occur due to increased blood volume. This can lead to decreased cerebral blood flow and cause dizziness.
Enlarging uterus impeding blood flow from the lower extremities: As the uterus expands, it can compress the inferior vena cava and pelvic veins, impeding blood flow from the lower extremities. This can cause dizziness and lightheadedness, especially when standing up from a supine position.

Small physiologic thrombi in the inferior vena cava: Although less likely, thrombi in the inferior vena cava can cause dizziness and lightheadedness due to decreased blood flow. This diagnosis is critical to consider, as it can be life-threatening if missed.
Venous insufficiency of the vertebral venous system: This is another important diagnosis to consider, as it can cause dizziness and lightheadedness due to decreased blood flow to the brain.

Cardiac abnormalities: Although rare, cardiac abnormalities such as cardiomyopathy or arrhythmias can cause dizziness and lightheadedness during pregnancy.
Neurological disorders: Rare neurological disorders such as multiple sclerosis or peripheral neuropathy can also cause dizziness and lightheadedness. However, these diagnoses are less likely in this patient given the absence of other symptoms.