Exposure to estrogen.  Because the female hormone estrogen stimulates breast cancer growth, exposure to estrogen over long periods of time, without any brakes, can increase the risk of breast cancer.  Some of these risk factors are not under your control, such as:  starting menstruation (monthly periods) at a young age (before age 12)  going through menopause (end of monthly cycles) at a late age (after 55)  exposure to estrogens in the environment (such as hormones in meat or pesticides such as DDT, which produce estrogen-like substances when broken down by the body).  Pregnancy and breastfeeding.  Pregnancy and breast feeding reduce the overall number of menstrual cycles in a woman’s lifetime, and this appears to reduce future breast cancer risk.  Women who have never had a full-term pregnancy, or had their first full-time pregnancy after age 30, have an increased risk of breast cancer.  For women, who do have children, breastfeeding may slightly lower their breast cancer risk.  DES exposure.  Women who took a medication called diethylstilbestrol (DES) used to prevent miscarriage from the 1940s through the 60s, have a slightly increased risk of breast cancer.  Women whose mothers took DES during pregnancy may have a higher risk of breast cancer.  Source NCI 2019