Risk Factors

Over-Exposure to Estrogen

Because growth of breast tissue is highly sensitive to estrogens, the more a woman is exposed to estrogen over her lifetime, the higher the risk for breast cancer.

Role of Estrogen Metabolism. A 2000 study suggested that the chance of estrogen increasing breast cancer risk in premenopausal women is related to how it is metabolized. In some women, very powerful estrogen products, or metabolites, are generated when metabolism takes place at a site on the estrogen molecule called C-16. These metabolites appear to pose a higher risk for breast cancer. (This metabolic effect does not appear to occur in postmenopausal women.) Fortunately, the study suggested that healthy diet and exercise may be able to alter this process.

Timing of Estrogen Exposure. Women's risk for breast cancer appears to be greater at specific times of estrogen exposure. For example, there is some evidence that starting one's period at an early age may be protective, in spite of the fact that this indicates a longer lifetime duration of estrogen exposure. Higher exposure in the womb (perhaps suggested by high birth weight), during pregnancy, or at menopause, however, does appear to increase risk.

Pregnancy and Abortion. Over the long term, women who have given birth even once have a lower risk than those who have not given birth. (Additional births do not seem to have any added impact.) However, there may be a higher risk for breast cancer in the immediate years after birth, particularly in older women.

Although a few studies have suggested a slightly increased risk for breast cancer in women who have had abortions, the weight of evidence does not support an association between abortion and breast cancer. However, interrupting a pregnancy does reduce the protective features of a full-term pregnancy.

Oral Contraception. Studies have been conflicting about whether estrogen in oral contraception increases the chances for breast cancer. A 2002 study supported an earlier major study, with both finding no evidence that OC use increases the risk for breast cancer, even in women who have taken them for 15 years of more or had taken them at young ages. In contrast, other studies have reported a higher risk in women who are current or recent users and in women who take them for more than four years before a first full-term pregnancy. Still, the risk for women taking OCs around menopause is unclear. Earlier research found a higher risk in women who used earlier forms of the pill containing high-dose estrogens and progestins (before 1975) and who had a family history of breast cancer.