Factors Associated with Decreased Risk of Breast CancerExercise
Based on solid evidence, exercising strenuously for more than 4 hours per week is associated with reduced breast cancer risk.
Magnitude of Effect: Average RR reduction is 30% to 40%. The effect may be greatest for premenopausal women of normal or low body weight.
Study Design: Prospective observational and case-control studies.
Internal Validity: Good. Consistency: Good. External Validity: Good.
Early pregnancy
Based on solid evidence, women who have a full-term pregnancy before age 20 have decreased breast cancer risk.
Magnitude of Effect: 50% decrease in breast cancer compared to nulliparous women or those who give birth after age 35.
Study Design: Cohort and case-control studies.
Internal Validity: Good. Consistency: Good. External validity: Good.
Breast-feeding
Based on solid evidence, women who breast-feed have a decreased risk of breast cancer.
Magnitude of Effect: The relative risk of breast cancer is decreased 4.3% for every 12 months of breast-feeding, in addition to 7% for each birth.[1]
Study design: Cohort and case-control studies.
Internal validity: Good. Consistency: Good. External validity: Good.
source:National Cancer Institute
