Final answer:
A CST places a rolled pad under the right hip of a pregnant surgical patient in the supine position to prevent vena cava compression, avoiding decreased blood flow back to the heart.
Step-by-step explanation:
When placing the pregnant surgical patient in the supine position, the Certified Surgical Technologist (CST) should place a small rolled pad or sheet under the right hip to prevent compression of the vena cava. This is essential because the growing uterus can compress the inferior vena cava when the patient is in the supine position, potentially leading to decreased venous return to the heart and hypotension, also known as supine hypotensive syndrome.