Factor	Comments
Factors associated with decreased risk
Bilateral oophorectomy before 50 years of age*	RR is reduced 50 percent compared with women who have never had the procedure5,6
Breastfeeding*	RR is reduced 4.3 percent for every 12 months of breastfeeding7,8
First full-term pregnancy before 20 years of age*	Risk is reduced in women who are younger at time of first delivery (e.g., in women who are approximately 60 years of age at the time of breast cancer diagnosis, the OR is 0.68 if they delivered their first child at age 20 years versus 0.79 if they first delivered at age 35 years)9
Menarche at or after 14 years of age	Risk is reduced 10 percent per two-year delay in menarche after 12 years of age10
Parity	Risk progressively decreases with increasing number of full-term pregnancies (e.g., OR of developing breast cancer after one full-term pregnancy is 1.0 versus 0.70 after five full-term pregnancies)9,10
Physical activity*	RR is reduced as much as 30 percent (dose-dependent) in premenopausal women11,12
	RR is reduced 11 to 22 percent in postmenopausal women, depending on exertional level and leanness11,12
Use of selective estrogen receptor modulators*	Risk is reduced; amount of reduction depends on initial risk stratification13–15
Factors associated with increased risk
Age	Cumulative incidence is 1.8 percent at 50 years of age
	Cumulative incidence is 3.8 percent at 60 years of age
	Cumulative incidence is 6.3 percent at 70 years of age
Alcohol intake*	RR is increased 7.1 percent per drink consumed beyond one drink per day16
Biopsy findings	Proliferative with atypia: risk is increased 4.24-fold17
	Proliferative without atypia: risk is increased 1.88-fold17
BMI*	Risk increases with increasing BMI; risk is increased 1.59-fold in women with BMI greater than 31 kg per m2; postmenopausal weight gain of more than 20 kg increases risk 1.99-fold18,19
Established BRCA1 or BRCA2 mutation	Risk is increased; exact level of risk is unknown20–23
Family history of breast cancer	Lifetime incidence of breast cancer is increased 5.5 percent in women with one affected first-degree relative and 13.3 percent in women with two22
History of ovarian cancer	Risk is increased
Hormonal exposure	Hormone therapy: combination therapy with estrogen plus progesterone for more than five years increases risk more than the use of estrogen alone. Risk decreases five years after discontinuing therapy 24,25
	Oral contraceptives: risk may be slightly increased with any use (OR, 1.19; 95% CI, 1.09 to 1.29), especially in parous women who use oral contraceptives for at least four years before their first full-term pregnancy (OR 1.52, 95% CI, 1.26 to 1.82)26,27
Increased breast density	Risk is increased28
Ionizing radiation	Significant risk exists, especially in youngest treatment cohorts18
Menopause after 45 years of age	Risk is increased about 3 percent for each year menopause is delayed10
Factors with unknown or no apparent effect
Benign breast lesion	No effect in women with nonproliferative patterns17
Diet	Coffee, tea, and other caffeine-containing drinks: no effect29
	Dietary phytoestrogens: no consistent effect30
	Fat: no consistent effect31,32
	Fruits and vegetables: no consistent effect33–35
Medication use	Antibiotics: no consistent effect, unlikely36–40
	Aspirin and nonsteroidal anti-inflammatory drugs: no consistent effect41–45
Miscarriage and induced abortion	No effect46
Smoking	No consistent effect16