In a study conducted on identical and fraternal twins (where one or both had breast cancer), researchers found that for identical twins with cancer, the first twin to reach puberty was five times more likely to get breast cancer first. If menstruation had begun before the age of 12, the chance for cancer was even stronger.

• According to the New England Journal of Medicine, a study was conducted on 1,811 sets of identical and fraternal female twins. In each set of twins, one or both of the twins had breast cancer. The researchers of the study looked for any patterns with their age at puberty and menopause, pregnancies and any other risk factors. The researchers found that for identical twins with cancer, the first twin to reach puberty was five times more likely to get breast cancer first. If menstruation began before the age of 12, the link was even stronger. Factors such as a later age for menopause, fewer children and a later first pregnancy made no difference in the results of this study.¹

¹Genes, Hormones, and Pathways to Breast Cancer, The New England Journal of Medicine, June 5, 2003, pp. 2352-2354.

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