There are several common genes, called polymorphic genes, that control the production, metabolism, and utilization of estrogens. We all have these genes; we just inherit different versions of them. Some of the polymorphic gene types occur in small percentages of the population, while others are very common. Some occur more often than others in certain racial or ethnic groups. This may represent older patterns of intermarriage and the resulting gene pool, or it may somehow be closely linked with genes that determine race. Several have been linked to slightly increased risk for breast cancer. Others have been linked to a predisposition to producing high levels of estrogens but may not be related per se to breast cancer risk. These genes may regulate how the body interacts with the environment in determining how hormones will affect the development and growth of breast tumors. Thus, study of these genes is a very important focus of breast cancer research today.

Pregnancy, Menstrual History, and Hormones
If you began menstruating at an early age (before age twelve), you have about a 30 to 40 percent increased risk of developing breast cancer compared with women who got their first menstrual periods after age sixteen. If you have never had a pregnancy lasting six months or more, or you had your first child after age thirty, you are at 50 to 100 percent higher risk for breast cancer compared with women who had their first full pregnancy before age twenty. Breastfeeding, on the other hand, seems to reduce your risk for breast cancer. Women who have breastfed for the longest total amount of time seem to have the greatest protection. In several studies, women who breastfed for six months or more reduced their risk for breast cancer by 30 to 40 percent compared with women who had never breastfed. If you stop menstruating at age forty five or younger, your risk of getting breast cancer is about half that of a woman who continues menstruating after age fifty five. This younger age at menopause is protective whether it comes about naturally or through surgical removal of the ovaries. (Hysterectomy or removal of the uterus without removal of the ovaries reduces breast cancer risk only very slightly. The important protective factor of early menopause seems to be the slowdown or removal of ovarian function.)

Blood Hormones
Women whose bodies continue to produce large amounts of estrogen (female hormones) and androgens (male hormones) after menopause have a much higher risk of breast cancer compared with women who produce low levels of these hormones. Although women’s ovaries slow down their hormone production after menopause, they continue to produce androgens, which then get converted into estrogens in other parts of the body, especially in fat tissue. Testosterone is an example of such an androgen. Since women who have passed menopause usually have a large amount of fat stores compared to muscle bulk, most of their blood estrogens come from what their body fat produces.
Several studies have measured blood levels of estrogen, androgens, and other hormones in women who eventually developed breast cancer compared with women who did not develop breast cancer. Most of the studies found that women who had high levels of any of the following hormones had an increased risk for developing breast cancer: estrogen, testosterone, other androgens, prolactin, insulin, and insulinlike growth factor. This last hormone is a protein that influences cells in the body to grow, including cancer cells.
Women whose bones are very dense or strong have a higher risk for breast cancer than women with less dense bones. Scientists believe that having highly dense bones is a marker for a lifetime exposure to high levels of estrogen. The estrogen keeps the bones stronger but unfortunately also has potentially harmful effects on the breast. This is not to say that having weak bones is a good thing! (Women with weak bones or osteoporosis have a high risk of sustaining fractures, including hip fractures.) Rather, high bone density may be a marker of increased breast cancer risk. If your doctor gives you a test to measure your bone strength (called a DEXA scan) and determines that you have very dense bones, you may also have high levels of blood estrogen. In this case, you might want to avoid hormone replacement therapy so that you do not add estrogen to your body.

Hormone Pills, Shots, and Creams
Taking postmenopausal estrogen replacement increases your risk for breast cancer by about 30 percent. Risk is highest if you have used estrogen therapy for more than five years. If you stop taking estrogen, your risk returns to that of the general population within five years of stopping. Estrogen therapy also increases the dense patterns seen on some women’s mammograms; these patterns are associated with increased breast cancer risk. These increased density patterns also make it more difficult for a radiologist to find small cancers, making the mammogram less sensitive at detecting cancer at an early stage in some women. If you have a high risk of developing breast cancer, or if you have had breast cancer, most doctors would advise you to avoid postmenopausal estrogen. Some researchers believe that adding the hormone progesterone to estrogen further increases your risk for breast cancer.
Many women are taking “natural” hormones, or herbs or soy derivatives that have hormonelike properties. Even though you may think that “natural” means safe, you should be aware that we do not know what are the long term effects of these substances. Some of them may compete with estrogens and may therefore reduce estrogen exposure of breast cells and protect against breast cancer. Others may act just like estrogens and could therefore increase risk for breast cancer. Since so little is known about these compounds, if you are seriously concerned about your risk for breast cancer or are at high risk for breast cancer, you should limit your intake of them and take them for no longer than five years. Small amounts of soy, such as including tofu in a varied diet, should not pose a problem. There are several studies just starting that will provide some information in the next five years or so regarding the breast safety of some of these substances.
Studies on breast cancer and oral contraceptives are still ongoing, but new evidence suggests that any risk from taking the pill is slight, if present at all. Any increase in risk that might be caused by oral contraceptives appears to be limited to women who have used oral contraceptives for many years, who began using them at an early age (i.e., in their teens), or who have a strong family history of breast cancer.