A women'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.
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 in life, according to the study from 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," said researcher Ann S. Hamilton. "This provides some more clues about a different approach in looking for genetic factors."
The findings appear in Thursday'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.
Since identical twins share genes, the researchers assume there was a hereditary reason behind the vulnerability to the onset of hormones. Scientists so far have discovered a few gene mutations that increase the risk of breast cancer.
While the focus has been on genes related to estrogen levels, Hamilton said, the study suggests also looking for genes that affect the sensitivity of immature breast cells at puberty.
Patricia Hartge of the National Cancer Institute, who wrote an accompanying editorial, said the study was provocative but its conclusions would have to be repeated in other studies. The study was partly funded by the institute.
"If we begin to understand how the hormone and gene go together to later increase the risk of breast cancer ... we probably could figure out how to intervene," she said.
Dr. JoAnn Manson of Harvard's Brigham and Women's Hospital said the implications of the study are 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, "There's even more impetus to try to reverse this epidemic of obesity in children," Manson said.