Final answer:
During pregnancy, the mother's cardiovascular system undergoes several changes. However, a slight decrease in resistance is not one of these changes.
Step-by-step explanation:
The change that does not occur in the mother's cardiovascular system during pregnancy is a slight decrease in resistance. During pregnancy, blood volume increases substantially, which helps manage the demands of fetal nourishment and waste removal. Additionally, the mother's pulse and blood pressure rise moderately due to increased blood volume and the uterus compressing pelvic blood vessels.