Girls are beginning puberty almost a year earlier than women 40 years ago, a new study suggests.
According to the NHS, puberty tends to begin between eight and 13 years in girls and nine and 14 years for boys.
However scientists have found the age of breast development, which represents the first clinical sign of female puberty, has shifted forward by about three months per decade since the late 1970s.
Study looked at breast development as an indicator
Researchers from the University of Copenhagen looked at data from 38 different studies that used breast development to track the age of puberty onset.
Study co-author Dr. Alexander Busch said the health implications of these findings are not yet clear.
"There are not many studies out there concerning the implications of early breast development for women's lifelong health," Busch said.
Busch added that the team decided against looking at the age of first menstruation as a marker of the beginning of puberty because it often relies on participants recalling when their periods began and can begin later than other developments.
Instead they looked at the development of glandular breast tissue, which is also known as thelarche.
Varied results worldwide
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 suggests the age of such changes is getting younger, with onset starting 0.24 years earlier per decade from 1977 to 2013.
They 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 researchers could not say why this is happening, but they pointed to a few possible contributing factors.
"Higher body mass index (BMI) is associated with the onset of menstruation and earlier breast gland development, and so it could play a part", Busch said.
Other factors such as endocrine-disrupting chemicals in the environment also might influence the age of puberty.
This study was published in the journal Jama Pediatrics.