NEW YORK—Women who reach puberty early have a higher risk of developing breast cancer than those who reach it late. So as the age of puberty's onset among U.S. girls continues to drop, researchers are trying to figure out why—and how this growing risk factor might be avoided. 

Although mutations on BRCA genes can increase a woman's chances of developing breast cancer, only 5 to 10 percent of breast cancer cases are linked to inherited mutations, according to the American Cancer Society. Even for those who have the BRCA mutation from one parent, "having one normal gene should be enough" to prevent the cancer, said Alisan Goldfarb, an assistant clinical professor at Mount Sinai School of Medicine, at a symposium presented here today by the hospital's Children's Environmental Health Center.

With more than a quarter million U.S. women diagnosed with breast cancer this year, however, it appears that additional factors are at work, leading Goldfarb to conclude that "breast cancer is made, not born."  

Unlike inherited genetic defects, "environmental factors can be discovered and prevented," Philip Landrigan, chairman of Mount Sinai's Department of Preventive Medicine, said at the event.

Both puberty and breast cancer are linked to increased levels of estrogen, and as research reveals more about estrogen-mimicking compounds (a group of endocrine disruptors that includes bisphenol-A and phthalates), some doctors and advocates are drawing more connections among the chemicals, our reproductive developmental phases and the disease.

These common compounds, which can thwart normal cell communications about growth and metabolism, have been a popular target of environmental health advocates campaigning against a variety of ills—from behavioral problems to reproductive issues—but they are gaining popularity in the breast cancer research field. Both the National Cancer Institute and the American Cancer Society, however, note that environmental factors such as these have yet to be proven as risk factors for breast cancer. 

Research on bisphenol-A (or BPA, commonly found in some plastics and canned goods) and phthalates (present in many personal care products and some food packaging) has shown that these classes of chemicals are common throughout the population, with the highest levels in children ages 6 to 11. These findings have led some in the field, such as Maida Galvez, an assistant professor of preventive medicine at Mount Sinai, to posit that endocrine disruptors such as bisphenol-A and phthalates might have a role in lowering the age of puberty in girls.

The average age of first menstruation in the U.S. has held stable for the past few decades (at about 12 years old). Menarche before that age increases the risk of breast cancer by about 30 percent, Galvez noted at the symposium. But what has begun to concern many in the field is an even earlier age of breast development, which dropped from 10.88 years old to 9.86 years between 1991 and 2006, according to a study published this April in Pediatrics. A recent national cohort study showed that some 13 percent of girls had already begun the second stages of breast development by age 7.

"I'm starting, in my own practice, to talk with families about changes in the body as early as 6 years old," said Galvez, who is also a pediatrician at the hospital. She proposed that during earlier development, breasts and the body overall might be more susceptible to environmental risks that could lead to cancer down the road.

The lines connecting these disparate findings are still tenuous, but studies are continuing to bring in new data. In the meantime, many parents are avoiding bringing more plastics and phthalate-containing products into the home—and many researchers are wondering what should be done about all of the BPA- and phthalate-laced equipment in labs and hospitals. 

Image of a mammogram of a breast with a cancerous formation courtesy of the National Cancer Institute via Wikimedia Commons