Reproductive hormones are thought to influence breast cancer risk by increasing cell proliferation, thereby increasing the likelihood of DNA damage, as well as promotion of cancer growth. Early menarche (<12 years) and older age at menopause (>55 years) may increase a woman’s risk of breast cancer by increasing lifetime exposure to reproductive hormones produced by her body.  Postmenopausal women with high levels of endogenous hormones (estrogen or testosterone) have about twice the risk of developing breast cancer compared to women with the lowest levels.  Few studies have examined this relationship in premenopausal women, and the results are mixed.

One reason for the discrepancy may be due to the complexity of measuring hormone levels in premenopausal women because
levels vary throughout the menstrual cycle. Younger age at first full-term pregnancy (<30 years) and a greater number of pregnancies decrease the risk of breast cancer over the long term; however, there also appears to be a transient increase in breast cancer risk following a full-term pregnancy, particularly among women who have a first birth at older ages.  Interestingly, recent studies suggest that reproductive patterns are more strongly associated with risk of hormone receptor-positive breast cancer compared to triple-negative breast cancer.

Breastfeeding has been shown to decrease a woman’s risk of breast cancer, with greater benefit associated with longer duration. In a review of 47 studies in 30 countries, the risk of breast cancer was reduced by 4.3% for every 12 months of breastfeeding.  Recent use of oral contraceptives may slightly increase the risk of breast cancer; however, women who stopped using oral contraceptives for 10 years or more have the same risk as women who never used the pill.

Recent use of menopausal hormones (previously referred to as hormone replacement therapy [HRT] or menopausal hormone
therapy) with combined estrogen and progestin increases the risk of developing and dying from breast cancer, with higher risk associated with longer use.  Risk is also greater for women who start hormone therapy soon after the onset of menopause compared to those who begin use later.  The increased risk appears to diminish within 5 years of discontinuation of hormone use.  Estrogen alone can be prescribed for women without a uterus, and it is less clear if this therapy increases risk of breast cancer. Results from the Women’s Health Initiative study suggest there is no increased risk of breast cancer associated with estrogen-only therapy.  However, several other studies found slight increases in risk associated with use, particularly among lean women and for women who began therapy soon after menopause.


Jeremy Baca pink breast cancer