The seemingly eternal irony: When we’re young we can’t wait to grow up; once we’re grown, we long for childhood, or at least aspects of it. Well, some populations of girls seem to be getting their wish — looking womanly at a girlish age. The latest group: preteens in Denmark, where breast development is beginning a year earlier, on average, than in the early ‘90s.

Lise Aksglaede and her colleagues at Rigshospitalet (Kingdom’s Hospital) in Copenhagen investigated evidence of nascent puberty in local girls — first in 1991 to ’93, and again from 2006 to ’08. And in the May Pediatrics, they report finding that every physical aspect of puberty they investigated — breast and pubic hair development and when a girl started getting her periods — occurred earlier, on average, in the more recent group of girls studied.

The rate at which age declined over time for each feature was not consistent. Breast development in the girls studied one to three years ago started at about not quite 11 years old — one year earlier than in the early 1990s. By contrast, girls who were studied in this decade started getting their periods at about age 13 years, 1.5 months old — or about three months earlier than in the early ‘90s.

If girls aren’t exiting puberty much earlier than before, Aksglaede and her colleagues argue, then the net effect has been a lengthening in puberty.

Of particular interest, the researchers note, two reproductive hormones that normally ratchet up to orchestrate the timing and manifestations of puberty — serum follicle-stimulating hormone and leuteinizing hormone — did not differ between girls at either end of the 15-year span. That suggests outside influences may be trumping the body’s reproductive hormones, the researches say.

Aksglaede’s group doesn’t have data on that but cites work by others who have demonstrated provocative hints of puberty-modifying features, from obesity to stress. A 70-page report that the Breast Cancer Fund issued in August 2007 reviews the biological basis for how these and other agents, such as lead, tobacco smoke and hormones in the food supply, might affect reproductive maturation in young women.

Indeed, native American girls in New York State and Quebec developed their first period at a later age than normal for their tribe if they had been exposed to relatively high concentrations of lead, a provocative 2005 paper in Pediatrics reported. In contrast, it noted, periods started at a younger age among girls who had been exposed in the womb to estrogen-mimicking polychlorinated biphenyls.

Endocrinologists, those docs who specialize in hormones, are concerned not only about what’s triggering precocious puberty, but also what its long-term implications might be. They’re thinking about such things as a possibly heightened breast-cancer risk decades from now.

Having a daughter, I can attest to a parent’s nightmare: that a particularly young and emotionally immature girl will be flattered by the attention of older guys (eyes glued to her chest) and persuaded to engage in sexually risky behaviors.

I’m not alone in these concerns. A 2006 editorial  (titled Adults at 12?) in the Journal of Epidemiology and Community Health pointed out that recent drops in the age of puberty’s onset “have not been matched by efforts to socially develop young people at an equally accelerated rate.” This is creating a growing disparity “between physical puberty and social puberty (the age at which people are mentally, educationally and legally equipped to function as adults in modern societies),” according to the authors, from Liverpool John Moores University in England. This social-puberty lag actually “may underpin many of the major public health challenges associated with young people today,” they concluded — from exaggerated sexual curiosity to especially risky and aggressive competition to garner social status.

Another 2006 paper, this one by environmental anthropologist Elizabeth A. Guillette of the University of Florida and her colleagues, correlated accelerated breast development in rural Mexican girls with elevated exposure to pesticides and other farm chemicals. In this study, however, the girl’s breasts weren’t really “developing” early in the normal sense of the term. They did get bigger, but the growth was largely due to deposition of fat, not the development of true breast tissue.

For ogling boys, the difference would be moot. But the authors note that this unusual fat development signals something anomalous — and not healthy. Moreover, they wrote in their Environmental Health Perspectives paper, this abnormal fattening in the breast area could be occurring in other populations as well, and largely missed if researchers base their categorization of breast development on outward appearances only.

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