Glossary

Type a term:

Reproductive History

Hormonal risk factors for breast cancer refer to exposures to natural hormones, particularly estradiol. Hormones are chemical substances that are secreted from glands and travel through the bloodstream to other areas of the body where they often stimulate some kind of physiological activity. Along with other hormones, estradiol is secreted by the ovaries over the course of the normal menstrual cycle.

Estradiol is also secreted increasingly during the latter weeks of pregnancy, along with a wide variety of other important hormones. Together, these hormones of pregnancy help the cells of the breast to fully differentiate and mature in preparation for milk production.

In general, the longer the breast tissue is exposed to estradiol, the higher the risk of breast cancer. Also, in general, the longer breast cells remain not fully differentiated, the higher the risk of the disease.

Menstrual Cycle

Chart

Early menarche, or first menstrual period before age 12, and late menopause, or menopause after age 55, both lead to small but significant increases in breast cancer rates because they result in a slightly prolonged lifetime exposure to estradiol. Extended exposure of breast cells to estradiol make proliferating cells more vulnerable to mutations in DNA as well as to changes in cellular functioning caused by environmental chemicals.

Reproductive history

Pregnancy and lactation are periods of increased estrogen exposure. Breast cells may be particularly susceptible to carcinogens during this time, especially during a woman’s first full-term pregnancy before her mammary tissue is fully matured. At the end of pregnancy, a woman’s mammary cells go through the final steps of development in preparation for breastfeeding. Fully developed cells are less sensitive to the effects of carcinogenic substances [Russo & Russo, 1998].

For women who have their first child before age 30, pregnancy affords some small, but significant, protection against breast cancer, especially for premenopausal women. The younger the woman is when she has her first child, the greater the risk reduction will be. On the other hand, having a first child at age 35 or older is associated with an increased risk of breast cancer [Merrill et al., 2005].

Breastfeeding (lactation) has also been associated with a decreased risk for breast cancer, with the longer a woman breastfeeds her child, the greater the protective effect [Collaborative Group on Hormonal Factors in Breast Cancer, 2002; Ursin et al., 2004]. Again, the effect is more pronounced for younger, premenopausal women.