How is breast cancer caused? Regarding the causes of breast cancer, the primary factor should be family history. If a first-degree relative has breast cancer, then the likelihood of developing breast cancer increases. The second factor is reproductive factors; for instance, if the age at menarche is quite young, menopause age is relatively late, menstrual cycles are short, there is no childbirth or the age at first full-term pregnancy is older, there are fewer childbirths, and there is a lack of breastfeeding, then the incidence of breast cancer is also higher. The third factor involves hormones, possibly related to endogenous hormones or exogenous hormones, such as those mainly associated with oral contraceptives used externally. The fourth major point is a high-fat diet, or habits like drinking alcohol, smoking, and a diet low in vitamins, which may increase the risk of breast cancer. The fifth point involves other factors, such as exposure to significant amounts of ionizing radiation, chemicals from cancer drugs, lack of physical exercise, and occupations, such as working in the beauty industry or pharmaceutical manufacturing, etc., which may also increase the chances of developing breast cancer.