A study of twins suggests early puberty may trigger the development of breast cancer in women already at unusually high risk because of their genetic makeup.

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, or never have children or have them late in life.

However, the new 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, according to the study by the University of Southern California at Los Angeles.

"There's a lot we don't know about the causes of breast cancer, but what we need to know... is where to look," researcher Ann S. Hamilton said. "This provides some more clues about a different approach in looking for genetic factors."

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 age at menopause, fewer children, and a later first pregnancy - made no difference.

JoAnn Manson of Harvard's Brigham and Women's Hospital said the study's implications were worrisome given the gradual decline in the age of puberty in the United States and the rise in childhood obesity. Body fat can stimulate hormones.

If the findings are correct, she said, "there's even more impetus to try to reverse this epidemic of obesity in children."