Final answer:
The correct answer is 'b. Cardiovascular system.' Pregnant women with twins experience significant changes and increased risk related to the cardiovascular system due to the substantial increase in blood volume and potential venous compression by the growing uterus.
Step-by-step explanation:
The nurse is developing a plan of care for a woman who is pregnant with twins and includes interventions focusing on the cardiovascular system because of the woman's increased risk. During pregnancy, blood volume increases substantially, by approximately 30 percent or 1-2 liters, to meet the demands of fetal nourishment and waste removal. Twin pregnancies further amplify these changes, with a higher blood volume expansion. Additionally, the growing uterus can compress underlying pelvic blood vessels, impacting venous return and contributing to conditions such as varicose veins and hemorrhoids. Therefore, special attention must be given to cardiovascular health to manage these risks effectively.