Girls are starting puberty almost one year earlier than 40 years ago, new research suggests.
According to the NHS, puberty tends to begin between eight and 13 years in girls and nine and 14 years for boys.
However, a new study has found that puberty in girls has shifted approximately three months earlier per decade since the 1970s.
Researchers from the University of Copenhagen assessed data from 38 studies involving tens of thousands of girls from around the world.
Each study examined when the development of glandular breast tissue – which is known in medical terms as “thelarche” and is a key marker of the onset of puberty – began to take place.
Dr Alexander Busch, the co-author of the research, said the work was the first to bring together and analyse studies focusing on thelarche.
Busch added that the team decided against looking at the age of first menstruation, or menarche, as a marker of the onset of puberty because it often relies on participants recalling when their periods began and can begin later than other developments.
Writing in the journal Jama Pediatrics, Busch and colleagues said they examined data from 38 studies which involved expert assessment of girls’ breast tissue.
Research looking at children with certain diseases, or who were severely malnourished or pathologically obese was excluded because these conditions can be known to affect the onset of puberty.
The team reported that development of glandular breast tissue varied around the world and over time, with studies finding an average age of onset between 9.8 and 10.8 years in Europe, compared with 10.1 to 13.2 years in Africa and 8.8 to 10.3 years in the US.
The team’s analysis also suggested that the age at which puberty begins is getting younger, with onset starting 0.24 years earlier per decade from 1977 to 2013.
While the research did not explore the reason as to why puberty might be starting earlier in girls, the team suggested that 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 added that a number of studies have previously suggested that chemicals in the environment could also interfere with the body’s hormone-based system and could therefore play a role.
Speaking to the Guardian, Busch said that the shift could be caused by a number of factors.
“It is important to proceed to monitor this as early puberty has implications,” he said.
“However, fighting childhood obesity and avoiding excessive exposure to environmental chemicals could help to avoid early pubertal onset.”
The study, published in the journal Human Reproduction, examined chemical groups including phthalates, parabens and phenols using urine tests. The researchers found that for some of these chemicals, every doubling of the concentration measured in the urine of pregnant women would cause their daughters to experience puberty on average a month earlier.
“We found evidence that some chemicals widely used in personal care products are associated with earlier puberty in girls,” said Dr Kim Harley, associate professor in public health at the University of California, who led the study.
“Specifically, we found that mothers who had higher levels of two chemicals in their bodies during pregnancy – diethyl phthalate, which is used in fragrance, and triclosan, which is an antibacterial agent in certain soaps and toothpaste – had daughters who entered puberty earlier.”