Girls are beginning puberty almost a year earlier than women 40 years ago, according to research.

Scientists have found the onset of development of glandular breast tissue has crept forwards by about three months per decade since the late 1970s.

Puberty generally begins between eight and 13 years in girls and nine and 14 years for boys. However, a number of global studies suggest the average age of puberty is falling.

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.

While the age of first menstruation, or menarche, decreased in the early 20th century, some research has suggested onset has shifted little in recent decades.

However, Busch and colleagues say looking at menarche as a marker of the onset of puberty is problematic, not least because it often relies on women recalling when their periods began, while menstruation begins later in puberty than other developments.

Instead the team focused on thelarche, with Busch noting the clinicians had seen a rise in the number of children under eight showing development of glandular breast tissue.

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.

Busch said many factors were probably at play: “It is important to proceed to monitor this as early puberty has implications. However, fighting childhood obesity and avoiding excessive exposure to environmental chemicals could help to avoid early pubertal onset.”

The study has some limitations, including that many studies did not report the body mass index of participants, and some studies were small.

However, the team says the new findings matter since they may mean that experts need to rethink current age cut-offs used to determine early puberty, to avoid healthy children being sent for brain scans and other tests.

Peter Hindmarsh, a professor of paediatric endocrinology at University College London, who was not involved in the study, said it was not yet clear whether the whole process of puberty in girls was shifting earlier, or whether it was only starting earlier and hence lasting longer.

“This trend towards an earlier start of puberty with breast development is interesting, but at this stage is not going to impact on how we approach early puberty and the definition of early puberty,” he said.