A history of early puberty may lead to the development of breast cancer in women who are already at unusually high risk because of their genetic makeup, a new study suggests. A woman's risk of breast cancer is believed to be linked to her lifelong exposure to the sex hormone estrogen, with slight increases for those who start menstruating early, reach menopause late, never have children or have them late.

The study suggests that going through puberty early may be especially ominous for some women.

For women genetically predisposed to get the disease, the rush of hormones at puberty alone -- rather than long-term exposure -- may result in breast cancer later in life, according to the study from the University of Southern California at Los Angeles.

The findings appear in today's New England Journal of Medicine.

The study looked at 1,811 sets of identical and fraternal female twins. In each set, one or both twins had breast cancer. The researchers asked about their age at puberty and menopause, pregnancies and other risk factors and looked for patterns.

One thing stood out: for identical twins with cancer, the first twin to reach puberty was five times more likely to get the disease first. The link was even stronger when menstruation began early, before the age of 12. Other factors -- a later menopause, fewer children and a later first pregnancy -- made no difference.

Since identical twins share genes, the researchers assume there was a hereditary reason behind the vulnerability to the onset of hormones.

While the focus has been on genes related to estrogen levels, researcher Ann Hamilton said, the study suggests also looking for genes that affect the sensitivity of immature breast cells at puberty.