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. Estrogenís job is to prepare a womanís body for pregnancy by promoting cell proliferation in the lobules and ducts of the breast (a precursor to milk production) and in the uterine lining (the endometrium). During a womanís period, estrogen levels rise following ovulation. But if no fertilized egg gets implanted in the uterus, estrogen levels fall and menstruation begins.
Estrogen by itself does not cause cancer ó the hormone, estrogen, it is an essential part of a womanís physiology. However, estrogenís principal function is to speed the process of cell proliferation in certain regions of the breast and uterus. Therefore, 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 an estrogen-dependent tumor. Exposure includes estrogen that your own body produces normally, as well as estrogen you might take as a pill (i.e.: birth control pills or HRT). 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)
Lengthy 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 mimics 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