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Why Are Girls Starting Puberty Earlier?

Fact: girls are starting puberty earlier. Breast development for instance, is occurring earlier and earlier and isn’t all the uncommon amongst 7-year olds. According to the most recent U.S. data (from 2013), 23 percent of black girls, 15 percent of Hispanic girls and 10 percent of white girls have started to develop breasts by the age of seven.

However, the age at which girls start to menstruate has begun on average only three months earlier compared with decades past. That means puberty not only begins earlier but lasts longer than before. This longer exposure to estrogen causes some alarm, since this could possible increase the risk of breast cancer, but the biggest issues are psychological.

The big question, then, is what’s causing this. Why are girls starting puberty earlier?


Obesity is definitely a factor researchers are looking at. The body needs to have a certain weight and fat percentage for puberty to be triggered, so the thought that early onset puberty is caused by childhood obesity seems a logical one. After all, the rate of obesity has more than doubled in American children over the past 30 years. Whereas only 7 percent of children aged six to 11 were obese in 1980, nearly 18 percent were obese in 2012.

However, there are more factors at play. One study showed for instance that Danish girls with the same BMI as American girls still hit puberty a full year later. A comparison of rich South African girls with African American girls showed the same gap. Other studies show that puberty is starting earlier in non-obese girls as well, though the differences are smaller. So obesity alone doesn’t explain early onset puberty.

Melatonin Levels

One hormone researchers have looked at is melatonin. This is the hormone in our bodies that regulates out internal clock and calendar. Kids naturally have higher melatonin levels, which explains why they sleep more and deeper than adults. But active kids produce more melatonin than sedentary kids. Over the last decades, kids have become way more sedentary, causing their melatonin levels to decrease. This could affect puberty, according to researchers, since melatonin acts as a puberty inhibitor: the more melatonin you have, the later you start puberty.


But researchers are looking to other factors as well, and chemicals are at the top of their list. A 2012 analysis by the U.S. Centers for Disease Control and Prevention (CDC) found for instance that American girls exposed to high levels of common household chemicals had their first periods seven months earlier than those with lower exposures. The culprits are all suspected to be endocrine disruptors: chemical pollutants like PCBs (polychlorinated biphenyls), phthalates (commonly used plasticizers) and certain pesticides.

PCB’s have been banned a while ago. They were used as cooling fluids and in copying machines for instance, but were proven to be carcinogens for animals and likely for humans as well. Even when it doesn’t cause cancer, it can disrupt the function of the thyroid, which is one of the key triggers for the menstrual cycle. Unfortunately, many rivers, parks, and buildings have already been contaminated with PCB’s and it’s shown up in food as well.

Phthalates are a group of chemicals used to soften and increase the flexibility of plastic. They show up in many personal care products, like shower gel, nail polish, and shampoo, but also in hundreds of often-used products like shower curtains, food packaging, toys, etc. The exact effects of phthalates are still being researches, but some have been shown to cause cancer in animals and others seem to affect the reproductive system, cause birth defect and disrupt the hormone levels in the human body.

The idea is that these chemicals and maybe others as well are disrupting the female bodies’ hormone balance and ‘trick’ the body into going into puberty. And because our bodies store these chemicals in fat cells, obese girls would have more chemicals in their bodies and thus experience the effects more. None of this has been proven scientifically though, though studies are ongoing. In the meantime, researchers are warning to limit exposure to these chemicals.

Growth Hormones

Another possible cause was the presence of growth hormones in for instance milk or meat. Could this affect the human body? Research has shown this is likely not the case, as these added hormones are quickly degraded in the stomach after consumption.

In short: there’s no clear single cause for why girls are starting puberty earlier. In all likelihood, it’s a combination of triggers and causes, with obesity and chemicals at the top of the list. What matters for us, as youth workers, is that we are aware of this trend and offer guidance where necessary. The biggest effects of hitting puberty early are psychological, with depression being a huge risk. Supporting girls who are going through this, then, becomes of crucial importance.

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