Scientific studies that look at risk suggest that the longer lifetime exposure to estrogen, the greater the chance that breast cancer can develop. The earlier in life that a girl begins to menstruate and the later in life that a woman enters menopause, the more hormones she is exposed to and the greater the odds are that she may develop breast cancer. Hormones occur naturally and regulate body functions. They tell glands and organs what to do and when to do it. The amount of hormone that a woman's body makes varies over time and depends on her body size, when she becomes sexually mature and when she enters menopause.

Hormones are not bad for a woman unless you have more than your body needs. When it comes to breast cancer, excess estrogen and progesterone exposure can increase one's risk of the disease. These hormones can cause tumors to grow larger and faster. Although your body makes estrogen naturally, chemicals that act like estrogen (called hormone-mimicking compounds, or endocrine disrupting compounds, EDCs) can be found elsewhere.

The use of low dose birth control pills and patches does not appear to be linked with breast cancer risk. In contrast, the Women's Health Initiative has shown that hormone replacement therapy increases breast cancer risk, and the longer a woman uses hormones to treat the symptoms of menopause the greater the risk. On the other hand, short-term use of hormones after menopause, for only a few months, does not appear to increase breast cancer risk.

Products that we and our families use for daily hygiene, household chores and killing pests can contain chemicals that act like estrogen. Herbal remedies can also behave this way. Estrogen-like “hormone mimics” can get in the way of our body's ability to tell organs and glands what to do.

Cancer usually takes years to develop. The amount of chemicals that people are exposed to (dose) and when that exposure happens (timing) have major impacts on a person's health. People may be repeatedly exposed to small doses of many different chemicals over time. For this reason, it can be difficult to determine the role of any single exposure in causing cancer.

When an exposure happens (over the course of a lifetime) can also affect whether a person gets sick or not. Research suggests that a woman's breasts are more sensitive to exposures during certain times of life - the prenatal period (before birth), adolescence (before puberty), pregnancy, and at the start of menopause. Exposures to toxic chemicals during these times of life may increase the chance a woman will get breast cancer later on.