Final answer:
During pregnancy, the cardiovascular system undergoes the most dramatic changes, including an increased blood volume and moderate rise in pulse and blood pressure.
Step-by-step explanation:
The body system of a pregnant woman that undergoes the most dramatic physiological changes during pregnancy is the cardiovascular system. The major changes include a substantial increase in blood volume by about 30 percent, or approximately 1-2 liters, to meet the demands for fetal nourishment and waste removal. Additionally, the pulse and blood pressure rise moderately, and the maternal cardiovascular system also has to cope with the growing uterus compressing pelvic blood vessels, which may lead to varicose veins or hemorrhoids.