Girls really are growing up quicker – biologically speaking.

The Guardian looks at the slightly puzzling change in the age girls reach puberty around the world:

For girls, experts say the best marker of the start of puberty is the development of glandular breast tissue, known as thelarche.

Researchers say they have reviewed studies on the milestone to reveal that puberty in girls has shifted, on average, three months earlier per decade from 1977 to 2013.

Dr Alexander Busch, the co-author of the research, from Rigshospitalet in Copenhagen, said the work was the first to draw together and analyse studies focusing on thelarche.

“That means also that there are not many studies out there concerning the implications of early breast development for [girls’] lifelong health,” he said, adding that early onset of menstruation was linked to a higher risk of conditions including obesity and cardiovascular disease.

Writing in the journal Jama Pediatrics, Busch and colleagues say they examined data from 38 studies published before mid-2019 that involved expert assessment of girls’ breast tissue.

The team excluded research looking at children with certain diseases, or who were severely malnourished or pathologically obese, because these conditions may affect the onset of puberty.

The team found that development of glandular breast tissue varied around the world and over time, with studies reporting an average age of onset between 9.8 and 10.8 years in Europe, depending on country and year, compared with 10.1 to 13.2 years in Africa and 8.8 to 10.3 years in the US.

The team’s analysis suggests the age of such changes is getting younger, with onset starting 0.24 years earlier per decade from 1977 to 2013.

While the research does not explore why puberty may be starting earlier in girls, the team say a higher body mass index is linked to earlier development of glandular breast tissue.

“The ongoing global obesity epidemic could partially explain the observed change in age at pubertal onset assessed as age at thelarche,” the authors write.

However, they also say a number of studies have suggested that chemicals in the environment that may interfere with the body’s hormone-based system could also play a role.

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