Changes in the Reproductive System During Pregnancy. When you become pregnant, the part of your body affected first and the part that undergoes the most significant changes is the uterus. It increases to 20 times its original weight, and 1,000 times its initial capacity.our breasts must undergo many changes during pregnancy to produce milk. In the first two months of pregnancy, your breasts may feel sore or full. They enlarge, and veins may become visible. Striae can develop.