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Reproductive habits and women’s hormonal status have changed enormously over the past century. Age at menarche and the average number of births have decreased, mainly in high-income countries.

Reproductive risk factors for female breast and endometrial cancer are related to the levels of estrogens in the body. Having menarche at a younger age increases the number of years the breast tissue is exposed to high levels of estrogens.

Earlier age at menarche is associated with increased risk of certain cancers; during the twentieth century, age at menarche decreased in many high-income countries.

Trends in age at menarche in Norway and the USA

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Bar graph showing trends in age at menarche in Norway (1918 compared to 1964) and the USA (1914 compared to 1970) which indicates that during the twentieth century, age at menarche decreased

Reproductive habits and women’s hormonal status have changed enormously over the past century.

Women who have their first full-term pregnancy at an early age have a decreased risk of developing breast cancer later in life, probably due to increased differentiation of the breast epithelial cells. The risk also declines with the number of children borne, and with breastfeeding for at least a year. Late menopause also increases the risk of breast cancer by increasing the duration of breast exposure to estrogen. A number of studies have suggested that current use of oral contraceptives causes a small and transient increase in the risk of breast cancer. However, the use of oral contraceptives causes a long-term and substantial reduction in the risk of endometrial and ovarian cancers.

Menopausal hormone therapy, often prescribed for short-term benefits, such as relief from hot flashes, is associated with a moderate increase in the risk for certain cancers including breast cancer, and the risk for breast cancer is greater for treatments containing both an estrogen and a progestogen. An IARC Monographs Working Group has concluded that combined estrogen-progestogen oral contraceptives and combined estrogen-progestogen menopausal therapy are carcinogenic to humans.

Breastfeeding and childbearing reduce the risk of breast cancer among women, whereas oral contraceptive use and hormone replacement therapy increase the risk.

Hormonal and reproductive risk factors for breast cancer

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Images with statistics and information that indicate that breastfeeding and childbearing reduce the risk of breast cancer among women, whereas oral contraceptive use and hormone replacement therapy increase the risk

“To expect that substantial reductions in breast-cancer incidence could be brought about today by women returning to the pattern of childbearing and breastfeeding that typified most societies until a century or so ago is unrealistic.

Collaborative Group on Hormonal Factors in Breast Cancer, 2002