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(Ivanhoe Newswire) -- A new study of twins is helping explain how exposure to ovarian hormones impacts the development of breast cancer in women.

Doctors have long known ovarian hormones stimulate the proliferation of cells in the breast and may increase the rate of genetic errors, both of which have been implicated in the development of breast cancer. However, many factors come into play, including age at puberty, number of children borne by the women, age at menopause, and others.

In this study, investigators examined data in about 1,800 pairs of female twins in which one or both members had been diagnosed with breast cancer. The twins were classified into several categories depending on whether one or both members had breast cancer, whether they were identical or fraternal, whether they had a family history of breast cancer, and whether they had breast cancer in one or both breasts.

Researchers say the most striking results were seen among the identical twins in the group. Among the pairs in which only one twin had breast cancer, suggesting a non-hereditary form of the disease, researchers found no link between an earlier onset of puberty and risk of the disease. However, in the pairs where both women had breast cancer, suggesting a hereditary form, earlier puberty was associated with an earlier diagnosis.

Researchers say other standard factors thought to be associated with greater hormone exposure over a lifetime, such as a later first pregnancy, fewer children, and later menopause, were significantly linked with breast cancer risk in pairs where only one twin had the disease. When both twins were affected, these factors did not predict an earlier diagnosis for one twin over the other.

Researchers believe these findings suggest most cases of hereditary breast cancer are linked, not to lifetime hormonal exposure, but to an unusual sensitivity to hormones during puberty. On the other hand, non-hereditary breast cancer appears not to be linked to earlier puberty, but to exposure to hormones over a woman's lifetime.

This article was reported by Ivanhoe.com, who offers Medical Alerts by e-mail every day of the week. To subscribe, go to: http://www.ivanhoe.com/newsalert/.

SOURCE: New England Journal of Medicine, 2003;348:2313-2322

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