Breast cancer may affect the patient’s menstruation because after having breast cancer, the growth of the tumor as well as when doing surgery and post-operative treatment will affect the menstruation as follows: 1. 2. Surgery: When patients undergo surgery on the breast, it will affect the patient’s body and also affect the patient’s menstruation. 3. Post-operative treatment: When patients undergo some treatment after surgery, it may also affect their menstruation, such as endocrine treatment or chemotherapy, etc. Especially when the patient does endocrine therapy, it will affect the patient’s ovarian function, so it will lead to prolonged menstruation or delayed menstruation, low menstrual volume, etc.