Girls are entering puberty about a year earlier than they were 40 years ago, according to a systematic review published Monday in JAMA Pediatrics.

Specifically, researchers found that the age at which girls begin to develop breasts has fallen by about three months per decade since the 1970s.

The review — which analyzed data from more than two dozen previous studies conducted in countries around the world — is different from most past ones. It focuses on the age at which girls develop glandular breast tissue (known as thelarche), which is considered the first physical sign of female puberty.  Previous studies have tended to look at the age at which girls experience their first menstrual period (known as menarche), which is considered the last sign of puberty for women.

The studies that have focused on first menstrual periods have also suggested that girls are entering puberty earlier, although some evidence indicates that most of the decrease may have occurred early in the 20th century rather than in more recent decades.

Menarche studies, however, tend to rely on women recalling when they had their first menstrual period, and those recollections may not always be accurate. The studies used in the current review relied on clinical information provided by doctors — data that is more objective, the authors of the review point out.

The lifelong health implications for women of the review’s findings — that breast development is occurring earlier — is unknown, as Dr. Alexander Busch of the University of Copenhagen, told HealthDay Reporter Dennis Thompson.

Early menarche, however, has been associated with health problems later in life, including an increased risk of obesity, type 2 diabetes, heart disease and allergies, Bush said.

A global phenomenon

For the review, Busch and his co-authors analyzed data from 30 studies that had assessed the age of the onset of glandular breast tissue development in girls. The studies were conducted between 1977 and 2013 and came from countries around the world, although most were from Europe or North America. The number of girls included in the studies ranged from 138 to 20,654.

The data revealed that the average age of breast development is currently around 10.1 to 13.2 years in Africa, 9.8 to 10.8 years in Europe and 8.8 to 10.3 years in the United States.

The researchers also found that across all the studies there was an overall decrease in the average age of breast development of 0.24 years — or about three months — per decade since the late 1970s. (Data from specific countries was not statistically strong enough to report the drop by geographic region, however.)

Possible explanations

The study didn’t look at why puberty is occurring earlier, but Bush and his co-authors mention two key possible reasons in their paper. One is the global obesity epidemic. Some research has shown that girls who are overweight tend to enter puberty earlier than girls who are lean, perhaps because of higher levels of the fat-regulating hormone leptin.

Another possible factor for why girls are entering puberty earlier has to do with the increasing amounts of endocrine-disrupting chemicals in the environment. A 2018 study found, for example, a link between early puberty and young children’s exposure to chemicals found in many brands of personal care products, such as toothpaste, soap and makeup.

“In addition, the banned but persistent chemicals such as DDT (dichlorodiphenyl-trichloroethane) and DDE (dichlorodiphenyldichloroethylene) have been associated with earlier age of puberty,” Busch and his co-authors write.

Limitations and implications

The study comes with caveats. Some of the studies were small, and not all them included the girls’ body mass index (BMI). In addition, studies that didn’t find a downward trend in the onset of puberty might not have been published. That phenomenon, known as publication bias, can skew the results of reviews like this one.

Still, these findings are important, Busch and his colleagues say, because the “assessment of breast development is widely used to distinguish between precocious and normally timed puberty.”

If the current age cutoff is too old, many healthy girls may be undergoing unnecessary medical tests.

“It is important to proceed to monitor this, as early puberty has implications,” Busch told Guardian reporter Nicola Davis. “However, fighting childhood obesity and avoiding excessive exposure to environmental chemicals could help to avoid early pubertal onset.”

FMI: You’ll find an abstract of the study at the JAMA Pediatrics website, but the full study is behind a paywall.