Girls who reach puberty too early have a higher chance of developing breast cancer. Scientists are scrambling to find out why puberty is arriving earlier and earlier for U.S. girls.

Although genes are a factor, a growing body of research is pointing to environmental factors, such as compounds commonly found in plastics that can mimic the female hormone, Scientific American reports. The suspected "endocrine disrupters" may be causing girls to reach puberty and develop breasts at younger ages.

"Breast cancer is made, not born," said Alisan Goldfarb, an assistant clinical professor at Mount Sinai School of Medicine, at a symposium presented by the Children’s Environmental Health Center in New York.

Genetic mutations account for only so many cases of breast cancer. Only 5 to 10 percent of breast cancer cases are linked to having a mutation of BRCA, or breast cancer gene, according to the American Cancer Society. But even for women who have the BRCA gene from one parent, "having one normal gene should be enough," said Goldfarb, who added that additional factors appear to be at work.

Indeed, more than a quarter of a million U.S. women will be diagnosed with breast cancer this year.

Fueling much of the research about a possible link between breast cancer and environmental factors is the knowledge that puberty and breast cancer are both linked to increased levels of estrogen. Now, some doctors are suggesting that estrogen-mimicking compounds, such as bisphenol-A and phthalates that are typically found in plastics or canned foods, may impact developmental phases and the disease.

BPA and phthalates are commonly found, but they are concentrated among children ages 6 to 11. Maida Galvez, an assistant professor of preventive medicine at Mount Sinai, suggested the compounds therefore might be acting as "endocrine disrupters" in lowering the age of puberty among girls.

In recent years, researchers have observed earlier breast development, which dropped to 9.86 years in 2006 from 10.88 in 1991, according to a study in the journal Pediatrics. Another recent study found that 13 percent of girls began the second stage of breast development at 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.

BPA and phthalates, which can interfere with cell growth and metabolism, have been blamed for other ailments, from behavioral problems to reproductive issues. The National Cancer Institute and the American Cancer Society say environmental factors are not proven risks for breast cancer, but more and more researchers are leaning in that direction.

"Environmental factors can be discovered and prevented," said Philip Landrigan, chairman of Mount Sinai’s Department of Preventative Medicine.