ages 40-74 have heterogeneously dense breasts and about 7% have extremely dense breasts (BI-RADS C or D, respectively).78 Breast density is influenced by genetics and other factors. For most women, breast density generally decreases with age, higher body weight, and after pregnancy and menopause.79 Some drugs also affect breast density, including tamoxifen (decreases density) and combined menopausal hormone therapy (increases density).79, 80 In early 2019, the US Food and Drug Administration proposed a federal rule that mammogram reports include information about breast density. Thirty-eight states and the District of Columbia previously passed some form of breast density legislation. Some state laws require that women with dense breasts be told that they may benefit from supplemental imaging tests, such as ultrasound or MRI. However, there is currently no expert consensus about what other tests, if any, should be done in addition to mammograms to screen for breast cancer in women with dense breasts.81 Ongoing clinical trials are evaluating whether digital breast tomosynthesis (DBT) or MRI may be more useful than conventional digital mammography in evaluating dense breasts.82 See page 20 for more information on DBT. Height Many studies have found that taller women have a higher risk of breast cancer than shorter women.83-85 A pooled study of more than 5 million women estimated that an increase of 10 cm (about 4 inches) in height was associated with about a 17% higher risk of breast cancer.84 Although the reasons are not fully understood, it may reflect differences in early growth as well as hormonal or genetic factors. Height is also associated with a number of other cancers, including colorectal and ovarian cancers. Menstrual cycles Breast cancer risk increases with earlier menstruation and later menopause.86 For example, breast cancer risk is about 20% higher among those who begin menstruating before age 11 compared to those who begin at age 14 or older. Likewise, women who experience menopause at age 55 or older have about a 12% higher risk compared to those who do so between ages 50-54.86 The increased risk may be due to longer lifetime exposure to reproductive hormones and has been more strongly linked to HR+ breast cancer than other subtypes.50 Bone mineral density High bone mineral density in postmenopausal women has been associated with a 60%-80% increased risk for breast cancer compared to low bone density; risk appears to be most strongly related to HR+ disease.87, 88 Bone density is not thought to be an independent risk factor for breast cancer, but a marker of cumulative estrogen exposure.89 However, because bone density is routinely measured to identify women at increased risk for osteoporosis (high bone density indicates absence of osteoporosis); it also may be helpful for identifying women at increased risk for breast cancer. Endogenous hormone levels Postmenopausal women with naturally high levels of certain endogenous sex hormones (e.g., estrogen, progesterone) have about twice the risk of developing breast cancer compared to women with the lowest levels, with the strongest relationships found for HR+ tumors.90, 91 16 Breast Cancer Facts & Figures 2019-2020 High circulating hormone levels are associated with, and may reflect, the effects of other breast cancer risk factors, such as postmenopausal obesity and alcohol use.91 Although it is challenging to study the relationship of hormones in premenopausal women because levels vary across the menstrual cycle, there is some evidence that high levels of circulating estrogens and androgens are associated with a small excess risk in premenopausal women, particularly for HR+ breast cancer.91-93 Reproductive factors Pregnancy Pregnancy has a dual effect on breast cancer risk.94 In the short term, women who have had a full-term pregnancy have an increased risk of both HR+ and HRbreast cancers that peaks at 5 years after childbirth. However, after about two decades, the relative risk of HR+ breast cancer becomes slightly lower (by about 20%-25%) in women who have given birth compared to those who have not. Risk is further reduced among women who have their first child at a younger age or have a greater number of children. In contrast, the increased risk for HR- breast cancer persists following a full-term pregnancy. Fertility drugs More research is needed on the relationship between breast cancer risk and the long-term effects of ovulationstimulating drugs.95 Most studies to date have found that breast cancer risk is not elevated in women who undergo in vitro fertilization.96-100 However, the data are less clear for clomiphene (Clomid), a drug that is often used as a first-line treatment for infertility.97, 99, 100 A long-term follow-up study of women seen at 5 US fertility clinics found no association with ever use of Clomid or gonadotropins; however, risk of invasive breast cancer was increased among women who underwent more than 12 Clomid treatment cycles compared to women who had never used fertility drugs.100 Another recent study from Norway reported that use of Clomid was linked to a slightly increased risk of breast cancer, but only among women who had given birth.99 Breastfeeding Most studies suggest that breastfeeding for a year or more slightly