For many girls in the developed world, puberty is coming earlier than ever before, with studies showing that, on average, puberty is now starting for girls at around 10 years old - at least five years earlier than a century ago.

There are several explanations for this, but research is now revealing a worrying side effect of the trend - early puberty seems to also increase the risk of health problems later in life, such as increased rate of breast cancer, heart disease, and depression, as Julie Beck reported for The Atlantic this week.

In other words, the hormonal changes associated with triggering early maturation seem to impact women long after puberty ends in ways we're only now beginning to understand. But what's going on here from a scientific point of view?

The idea that girls are reaching sexual adolescence earlier than ever before is nothing new - for years, researchers have noticed that women are getting their periods earlier and earlier - the average age for menstruation to start is now 13, compared to 16 or 17 at the turn of the last century.

They're also starting breast development earlier, too - which is the scientific start of puberty. A 2013 longitudinal study showed that, in the US, girls on average start growing breasts at 8.8 years of age if they're African American, and between 9.3 and 9.7 years of age if they're Asian, Hispanic, or white.

And for all groups but African Americans, that number was significantly lower than it had been 10 and 20 years ago.

Researchers think this change could be a result of our obesity epidemic - excess body weight has been shown to affect the timing of a girl's first period. But studies suggest that this weight gain doesn't entirely explain the extent to which puberty has moved forwards in girls.

Another idea is that our increased exposure to pollution and hormone-disrupting chemicals, such as phthalates, could be causing the premature changes, but, again, the data doesn't fully explain this association.

What is becoming clear, though, is that these changes have far-reaching impacts. A study published back in May looked at more than 8,327 children born in Hong Kong in 1997, and found that girls who started developing breasts earlier had a higher risk of depression when they were 12 or 15 years old.

"What we found was the girls who had earlier breast development had a higher risk of depressive symptoms, or more depressive symptoms," lead researcher of the study, C. Mary Schooling, from the City University of New York School of Public Health, told The New York Times. "We didn’t see the same thing for boys."

She explains that the link between depression and early puberty was apparent, even after controlling for other variables, such as socioeconomic status, weight, or parents' marital status.

Research has also shown a link between early puberty and an increased risk of breast cancer, heart disease, drug use, and even all-cause mortality - which is basically the chance of women dying of any reason.

In a 2013 paper published in the Journal of Adolescent Health, researcher Frank M. Biro from the University of Cincinnati found that girls who start their period younger than their peers have up to a 30 percent increased risk of breast cancer.

"For each year that age of menarche was delayed, the risk of premenopausal breast cancer was reduced by 9 percent, and risk of postmenopausal breast cancer was reduced by 4 percent," the study concluded.

It's important to note that correlation doesn't equal causation, and there's no evidence that it's the early puberty that's driving these trends. But that data clearly suggests that girls who start to develop at a young age have something going on that's increasing their risk of other diseases.

Biro suggests that it could be a result of the hormonal changes of puberty making girls' bodies more susceptible to stress. "Puberty is considered one of those windows of susceptibility," Biro told Susan Scutti from NewsWeek last year.

"In particular, the actively maturing breast tissue of a girl, unlike the breast tissue of a full-grown woman, is more vulnerable to damaging environmental pollutants," Scutti reports.

The increased risk of depression may also be associated with these hormonal changes, or could be explained by the extra stress, self consciousness, and sexual attention associated with early puberty.

"We do know that being a late bloomer or early bloomer and standing out from your peers can be a stress for some kids," Louise Greenspan, a paediatric endocrinologist from the University of California, San Francisco, and author of the book The New Puberty, told Scutti.

There's a lot more work to be done on this topic before we fully begin to understand what's going on in these girls' bodies, but the more we learn about the reasons behind early puberty, and the effects it has, the better chance we have of preventing and predicting some of its long-term risks.

And as the trend towards earlier sexual maturation continues, that's only going to get more important. Watch this space.