Despite the reassurance of pediatric endocrinologists that younger development is normal, a lot of parents are still nervous, Kaplowitz says.
"If somebody calls in and says, 'I've got an 8-year-old with breast buds,' there's nothing I need to do," he says. "I discourage referrals. But they show up anyway."
Kaplowitz examined evidence for all suspected environmental and lifestyle factors in his book, "Early Puberty in Girls: The Essential Guide to Coping With This Common Problem."
"The explanation for which there's the most evidence is that it's related to the trend in increasing obesity," he says. "There are other factors, such as if your mother matured early. Sometimes we simply don't know. But overall, the biggest single factor is the trend toward obesity." Fatty tissue is a source of estrogen, so chubbier girls are exposed to more estrogen.
"With environmental influences, there has been a lot of speculation, but little hard data. I'm not suggesting there's no connection, but it's very hard to say there's a proven connection. I think it's environmental mainly in the sense that overeating and lack of exercise is environmental," Kaplowitz says. "I've tried to take the view that we shouldn't be alarmed about this."
Herman-Giddens is not so convinced, but concedes that evidence for environmental causes is close to impossible to obtain. "I myself am shocked sometimes to see very thin girls, 8 and 9 years old, with breast development," she says. "But with all the estrogen-like elements in the environment, it's virtually impossible to study. There's no place to find an unexposed population."
The biggest concern, she says, is that earlier puberty means longer lifetime exposure to estrogen, and early puberty, along with late menopause, is known to increase the risk of breast cancer.
But to design a study in which some girls are deliberately exposed to higher doses of such chemicals would be unethical, she says. Some animal studies provide cause for concern about endocrine-disrupting chemicals, but little hard evidence for humans. And a handful of industrial accidents have provided some data. In 1973, for example, estrogenic chemicals were inadvertently mixed in cattle feed in a Michigan community. The daughters of pregnant and nursing women who ate meat and dairy products from the cows were studied and were found to have begun their periods up to a year earlier than girls not exposed to the chemical, according to a 2000 study in the journal Epidemiology.
