Reproductive and Hormonal Factors

Reproductive and hormonal factors, both endogenous (naturally synthesized in the body) and exogenous (man-made), play important roles in the development of common cancers, including breast, endometrial, ovarian, and cervical cancer, with the strength and direction of these associations varying by cancer types (Figure 9.1). Increased access to higher education, evolving social norms, and better availability of and access to contraception have markedly changed women’s reproductive patterns globally and, subsequently, the incidence rates of related cancer types.

Women are having fewer children (Figure 9.2) and delaying motherhood to older ages (Figure 9.3). The average age of menarche (first menstrual period) has been decreasing, while the average age of menopause (end of menstrual periods) has been increasing. Early menarche, late natural menopause, not having children, and first pregnancy after age 30 increase lifetime exposure to endogenous hormones, thereby increasing the incidence of breast and other hormone-related cancers (see Breast Cancer).

Although the protective effect of breastfeeding against breast cancer is well-documented, the current prevalence of breastfeeding in many high-income regions such as North America is lower than the World Health Organization target of 50% in 2023 (Figure 9.4).