reduces a woman’s overall risk of breast cancer, with longer duration associated with greater risk reduction. In a review of 47 studies in 30 countries, the risk of breast cancer was reduced by 4% for every 12 months of breastfeeding.101 The protective effect may be stronger for – or even limited to – triple negative cancers.102-104 Hormonal birth control Most studies have found that current or recent use of oral contraceptives (combined estrogen and progesterone) is associated with a small (about 20%) relative increase in breast cancer risk, particularly among women who begin use before first pregnancy.105, 106 Risk appears to diminish when women stop use, and after about 10 years, it is similar to those who have never taken oral contraceptives. Notably, data are limited and less clear for “ultra lowdose” (20 micrograms) estrogen formulations.107 Studies of the levonorgestrel-releasing intrauterine device (Mirena) have produced conflicting results, but a large study from Denmark found that use of Mirena also increases breast cancer risk by about 20%.105, 108-110 In contrast, the use of the injectable progestin-only contraceptive depot-medroxyprogesterone acetate (Depo-Provera) does not seem to be linked with breast cancer.105, 111 Overall, it has been estimated that one extra breast cancer is diagnosed for every 7,690 women using hormonal contraception for one year.105 Postmenopausal hormones Recent use of menopausal hormones (also referred to as hormone therapy or hormone replacement therapy) with combined estrogen and progestin increases the risk of HR+ breast cancer, with higher risk associated with longer use.50, 69, 112, 113 Risk appears to be greater for women who start hormone therapy soon after the onset of menopause compared to those who begin later.113, 114 Discontinuation of menopausal hormones diminishes but does not eliminate the increase in breast cancer risk.115 Combined hormone therapy also increases breast density.80 Breast Cancer Facts & Figures 2019-2020 17 Postmenopausal estrogen-only therapy has been associated with uterine problems (including endometrial cancer) and is therefore only given to women who have undergone hysterectomy. The effects of estrogen-only therapy on breast cancer risk is less clear, but they are likely minimal at most. The Women’s Health Initiative randomized trial116 found that women who used estrogen-only therapy for an average of 6 years had a 25% lower risk of developing breast cancer, but several observational studies have found a slight increase in breast cancer risk among estrogen therapy users, particularly among lean women and those who begin therapy soon after menopause.114, 117, 118 Conflicting results may reflect differences between studies in the prevalence of obesity or higher rates of screening in menopausal hormone users in the observational studies.119 Recently reported results after 18 years of follow-up of the Women’s Health Initiative randomized trial found no increased risk of death overall or due to breast cancer associated with use of estrogen plus progestin or estrogen alone.120 Excess body weight, physical inactivity, diet, alcohol, and tobacco Excess body weight and weight gain Postmenopausal HR+ breast cancer risk is about 1.5-2 times higher in women who are overweight or obese.121 Even within the normal range of BMI (18.5-24.9), higher levels of body fat are associated with increased risk of breast cancer after menopause.122 This is likely due, in part, to higher estrogen levels because fat tissue is the largest source of estrogen in postmenopausal women, but may also be related to other mechanisms, including the higher levels of insulin among women with excess body weight.122, 123 Weight gain also increases risk of postmenopausal breast cancer.124, 125 A large meta-analysis found that for each 5 kilograms (about 11 pounds) gained during adulthood, risk of postmenopausal breast cancer increases by 11%.125 Notably, the increased risk was only observed among women who did not use menopausal hormones. Weight loss in early adulthood and after menopause is associated with reduced breast cancer risk in some, but not all studies.123, 126 The effects of weight loss are more difficult to examine because it is often not sustained. In contrast, studies have found that excess body weight protects against premenopausal breast cancer. A large meta-analysis found that among women between 40 and 49 years of age, the risk for developing breast cancer was about 14% lower in overweight women and 26% lower in obese women compared to women who were normal weight.127 The underlying mechanisms for this inverse relationship are not well understood.123 Physical inactivity Women who get regular physical activity have a 10%-20% lower risk of breast cancer compared to women who are inactive, with greater risk reduction associated with increasing levels of activity.128-131 The protective effect is independent of BMI and may be limited to women who have never used menopausal hormone therapy.131 The benefit may be due to the effects of physical activity on systemic inflammation, hormone levels, and energy balance.131, 132 Diet Numerous studies have examined the relationship between food consumption (including fat, fiber, soy, dairy, meat,