What Are the Risk Factors for Breast Cancer (IV)?

Previous abnormal breast biopsy: Women whose earlier breast biopsies detected any of these changes have a slightly higher risk of breast cancer (1.5 to 2 times greater than other women):
• fibroadenoma with complex features
• hyperplasia without atypia
• sclerosing adenosis
• multiple papillomas
Atypical hyperplasia (ductal or lobular) increases a woman's breast cancer risk by 4 to 5 times.
Having a biopsy specimen diagnosed as fibrocystic changes without proliferative breast disease or fibroadenoma does not affect breast cancer risk.

Breast-feeding and pregnancy: Some studies suggest that breast-feeding may slightly lower breast cancer risk, especially if breast-feeding is continued for 1.5 to 2 years. Other studies found no impact on breast cancer risk.
The explanation of this may be that both pregnancy and breast-feeding reduce a woman's total number of lifetime menstrual cycles. This may be similar to the reduction of risk due to late menarche (start of menstrual periods) or early menopause, which also decrease the total number of menstrual cycles. One study concluded that having more children and breast-feeding longer could reduce the risk of breast cancer by half.

Menstrual periods: Women who started menstruating at an early age (before age 12) or who went through menopause at a late age (after age 55) have a slightly higher risk of breast cancer.
Diethylstilbestrol (DES): In the 1940s through the 1960s some pregnant women were given diethylstilbestrol because it was thought to lower their chances of losing the baby. Recent studies have shown that these women have a slightly increased risk of developing breast cancer.