Why Are 7- and 8-Year-Old Girls Entering Puberty? There’s new research.

In August this year, we learned that lots of seven- and eight-year-old girls are sprouting breasts. A study in the journal Pediatrics reported that 10 percent of white and 23 percent of black seven-year-olds, and 18 percent and 43 percent of eight-year-olds, are showing signs of sexual maturity.

It wasn’t the first time “precocious puberty” made headlines, but the numbers appear to be sliding even earlier. Last year in a Pediatrics study, it was reported that breast development started at an average age of almost 11 years in 1991 and 10 years in 2006. Indeed, most of the numbers out there indicate that girls in puberty have been getting progressively younger since the mid-1800′s. The trend used to signal a healthier population – the result of improved nutrition and lower incidence of disease. But that reasoning can only go so far.

It’s an unsettling idea – kind of creepy, actually – that something in our modern world could be messing with our kids’ biological clocks. Recently, a lot of explanations have been posed and red flags raised about the causes. Which ones should we pay most attention to?

Divorce or, more specifically, not having a biological father in the house was recently linked to early puberty. Absent dads in middle-class homes increased the risk of early breast development in girls ages six to eight. Family stress and insecure attachments spur on early sexual maturity, says Jay Belsky, a researcher who, in a separate study this year, reported that babies who didn’t smile, vocalize, and reach for mom (signs of secure attachment) started puberty earlier. Belsky’s explanation is that when times are tough, nature kicks in to start puberty earlier, because it ups the chance of a person mating and having offspring before it’s too late.

Other suspects include our old friends bisphenol-A (BPA) and phthalates. There is reason to think these compounds act on the body’s endocrine system, influencing how hormones are made and released. BPA (a building block of plastics) mimics estrogen, while phthalates (chemicals in soft plastics, cosmetics, and more) block the effects of testosterone and have been found to lower sperm count, for example, in male animals.

But if family stress and environmental toxins are scooching girls toward faster development, the effects are probably small. First of all, I’m not sure I buy that insecure homes speed up development, because as we’ve seen, when nutrition (one of the best indicators of a secure environment) improved, puberty age went down. And we can only speculate about endocrine-disrupting chemicals; yes we have some animal data, but it’s incredibly hard to sift apart the direct effects of one chemical on humans when we’re exposed to so many every day. One of the Pediatrics researchers is currently testing girls’ blood to look for more direct evidence of this link.

The most likely culprit of precocious puberty is the obesity epidemic. Overweight girls are 50 percent more likely to enter puberty early, and those considered obese have an 80 percent chance of developing breasts before their ninth birthday. In this country, nearly one third of children and teens are overweight or obese.

Why would fat signal the body to mature faster? Probably by tripping the hormone system that starts puberty in the first place. In the normal pre-teen brain, a grape-sized structure called the hypothalamus starts secreting the chemical GnRH, which then, by way of the pituitary gland, tells the ovaries and testes to make estrogen and testosterone, and in girls, “breast buds” are usually the first result. While this process is normal, chubbier children have more of the chemical leptin, which can stimulate the release of GnRH, as well as the pituitary’s hormones.

It’s important to point out that all these studies talk about the dropping age of breast development (a subjective measurement), but meanwhile the age of the most objective measure of puberty – the age of girls’ first periods – has stayed more or less stable, at just over 12 years, over the decades. No one knows what this means, but I take comfort in the fact that the most concrete marker of sexual development hasn’t budged.

Still there are plenty of reasons to be suspicious of toxins in our everyday products. The F.D.A. doesn’t require that manufacturers prove their safety before they are used — the burden of proof goes the other way around. But the body’s hormone system is complex, as is the combination of chemicals and other exposures our kids have every day of their lives, which makes it difficult to point the figure at one particular chemical.

Just to be safe, however, it probably makes sense to buy BPA-free bottles and be aware of phthalates – more products are labeling for this purpose, and why not hedge your bets? But in the trend towards earlier training bras for girls, our nation’s expanding waistline is probably the guilty party.

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