Puberty has always been a time associated with awkwardness, bad skin, and biological changes that can be jarring for even the most grounded adolescent. No one has put a lot of thought into how someone as young as 7 years old could handle these changes, but new research suggests we may have to start.

At the turn of the 20th century, the average girl would begin menstruating around the age of 16 or 17. By 2008, that age had fallen to less than 13. But even before a girl gets her first period, there are signs of maturation that signal impending changes, and these come even earlier. A generation ago, less than 5 percent of girls would see these shifts — breast growth, body hair — before the age of eight. Now, that percentage has nearly doubled, according to Newsweek. As if this weren’t shocking enough, some clinicians think this age is still falling. At Kaiser Permanente in Northern California, doctors see fit to begin assessing girls for puberty-related changes at age 6.

Classically, precocious puberty has defined puberty that begins before age 8 in girls and 9 in boys, but this is no longer universally accepted.

“In general, we think that 7 is now probably a normal age to have some signs of puberty,” Louise Greenspan, a pediatric endocrinologist at Kaiser Permanente, told The Wall Street Journal. “The cutoff for precocious puberty is a gray zone now.”

Not every researcher agrees puberty is beginning earlier and earlier, but there have been a plethora of studies supporting this idea. Now scientists are looking at the effects of precocious puberty, and trying to figure out why it is occurring.

What’s The Big Deal?

So far, researchers haven’t proven any physical risks that come with early maturity. Though large amounts of estrogen have been associated with an increased risk of breast cancer, there isn’t any evidence that early puberty exposes a girl to enough of the hormone to be a problem. But lately, many researchers have suggested that the main risks that come along with precocious puberty are not biological. One May 2016 study found that girls who began the process early had an increased risk of depression during their adolescent years. Some researchers also point to social risks that can disrupt a girl’s healthy development.

“We interact with girls as they appear,” Frank M. Biro, of the Cincinnati Children’s Hospital Medical Center, told Scientific American. “People relate to an early-maturing girl as if she is older than she is, but there is really no correlation between age of onset of puberty and one’s social or emotional maturation.”

This can be very confusing and emotionally damaging for girls, as they may face “sexual innuendo or teasing” long before they’re ready for it, according to the article. Early puberty may change the way a girl behaves, along with the way others behave towards her.

“We know that the early-maturing girls are at an increased risk of some of these risk-taking behaviors: alcohol use, smoking, drug use, and earlier engagement in sexual behaviors,” Biro told The Wall Street Journal.

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Why Is It Happening?

Many experts point to one of the biggest American public health issues as the culprit behind early puberty trends: obesity. Childhood obesity rates have more than quadrupled in the past 30 years, with more than one-third of children and adolescents weighing in as overweight or obese. Fat cells produce estrogen, which plays a central role in stimulating breast growth in girls.

The theory that obesity is leading to earlier puberty is supported by the fact that though girls’ breasts may be developing at a much younger age, the age at which they start to menstruate has declined by only about three months within the last few decades. The ovaries control menstruation, signaling that earlier breast development may be occurring because of a different variable. In other words, girls who enter precocious puberty tend to have a longer maturation process, beginning at a younger age.

There are other factors at play, though. Girls at a normal weight have been starting puberty earlier as well, though at a lower rate than their heavier peers, and that leaves researchers with questions. Some have pointed to chemicals known as endocrine disruptors, such as the phthalates used in the production of plastics, as another potential contributor to early puberty. They mimic estrogen and could therefore cause precocious breast growth. Others have said stress during childhood can play a role in prompting puberty as well.

“Girls who grow up in families with a lot of strife or violence in their neighborhood are more likely to develop earlier,” Greenspan said. Her research has suggested that girls who grew up without their biological father were twice as likely to get their period before age 12, according to The Wall Street Journal.

Scientists are even researching prenatal variables. One study found that overweight mothers who developed gestational diabetes while pregnant gave birth to daughters who would start puberty earlier in life, regardless of what the girls themselves weighed.

Regardless of whether its cause is environmental, genetic, biological, or some combination, precocious puberty may be reaching a biological breaking point.

“I don’t know if girls can go through puberty any earlier,” Biro said. “We may be approaching a biological minimum.”