Risk FactorsAccording to the [https://www.cancer.gov/types/breast/hp/breast-prevention-pdq National Cancer Institute], the major risk factors for breast cancer are '''female sex''' and '''increasing age'''. Women with '''dense breasts''' have an increased risk proportionate to the degree of density. Among women with a '''family history of breast cancer''', risk is doubled if a single first-degree relative is affected and risk is increased fivefold if two first-degree relatives are diagnosed. Also, the lifetime risk is 55% to 65% for '''BRCA1''' mutation carriers and 45% to 47% for '''BRCA2''' mutation carriers compared to a lifetime risk of 12% in the general population.
Reproductive risk factors: Women who have a '''full-term pregnancy''' before age 20 years have a 50% decrease in breast cancer risk compared with nulliparous women or women who give birth after age 35 years, and women who practice '''breast-feeding''' have a 4% decrease in risk of breast cancer for every 12 months of breast-feeding in addition to 7% for each birth. Also, undergoing oophorectomy or other forms of '''premature menopause''' may reduce breast cancer risk as much as 75% depending on age, weight, and parity, with the greatest reduction for young, thin, nulliparous women. Conversely, women who experience '''menarche at age 11 years or younger''' have about a 20% greater chance of developing breast cancer than do those who experience menarche at age 14 years or older.
Behavioral risk factors: [https://www-doh.state.nj.us/doh-shad/query/builder/njbrfs/AlcoholChrnHvy/AlcoholChrnHvyCrude11_.html Alcohol consumption] is associated with increased breast cancer risk in a dose-dependent fashion and '''obesity''' is associated with an increased breast cancer risk in postmenopausal women who have not used combination hormone therapy (although it is uncertain whether either reduced alcohol consumption among women who are heavy drinkers or weight reduction among obese women decreases the risk of breast cancer). Conversely, '''exercising strenuously''' for more than 4 hours per week is associated with an average RR reduction of 30% to 40%. (The effect may be greatest for premenopausal women of normal or low body weight.)
[Last reviewed: 1/26/20]
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