Estrogen is a female hormone which is most active in the breasts and uterus. Consequently, the tissues in the breast and uterus are very sensitive to the presence of estrogen. Estrogen’s job is to prepare a woman’s body for pregnancy which it does by promoting cell proliferation in the lobules and ducts of the breast—a precursor to milk production—and in the uterine lining or endometrium. During a woman’s period, estrogen levels rise following ovulation. But if no fertilized egg gets implanted in the uterus, estrogen levels fall and menstruation begins.
Estrogen in itself does not cause cancer—it is an essential part of a woman’s physiology. However, estrogen’s principal function is to speed the process of cell proliferation. So it follows that estrogen can increase the chance of a mutation occurring and/or encourage the growth of cancerous cells once they appear. The more estrogen a woman is exposed to during her lifetime, the greater the opportunity for the hormone to promote the growth of a tumor. This includes estrogen that your own body produces normally, as well as estrogen you might take as a pill (i.e. HRT, or birith control pills). The following risk factors for breast cancer stem from prolonged estrogen exposure:
• Starting menstruation at a young age (more years of the body producing estrogen).
• Starting menopause at a late age (more years of the body producing estrogen).
• Length use of Hormone Replacement Therapy.
• Never having had a full-term pregnancy,
• Having a first full-term pregnancy after age 30 (more years of the body producing estrogen without the break from regular cycles)
• Being overweight, which increases the production of estrogen outside the ovaries and adds to the overall level of estrogen in the body,
• Exposure to estrogens in the environment (such as estrogen fed to fatten up beef cattle, or the breakdown products of the pesticide DDT, which mimic the effects of estrogen in the body),
• Consuming more than two alcoholic drinks per week, which can limit your liver's ability to regulate blood estrogen levels.