Dr. Karen Hendershott, breast surgical oncologist at Arizona Oncology told Newsweek that most breast cancer treatments can be given during the second and third trimesters: "This includes surgery and most forms of chemotherapy," she said.
But some treatments can harm the fetus at any time during pregnancy, and most doctors try to avoid these until after the delivery. One of them is radiation therapy, which uses high-energy x-rays to destroy cancer cells; depending on the radiation dose and the area of treatment, it could harm the fetus throughout pregnancy. It can affect the support cells and blood supply of the uterus and increase the risk of miscarriage, among other problems.
Chemotherapy includes treatment that may damage heart cells and weaken the heart, forcing it to work harder during pregnancy and labor.