'New Normal or Result of Stress & Chemicals?' Puberty Starts Earlier Than It Used To, Study Claims

Puberty – the point in life when a person becomes sexually mature – typically occurs between ages 8 and 13 for girls and ages 9 and 14 for boys. However, there are shifts in this timescale on occasion. “Precocious” puberty refers to the development starting abnormally early, and delayed puberty, accordingly, begins unusually late.

The onset of puberty appearing to come earlier than it used to is increasingly raising concerns, with researchers looking into factors that might be impacting the process.

Thus, on average, girls in the mid-1990s started to develop breasts - typically the first sign of puberty - over a year earlier than previously recorded, a landmark study back in the 1990s revealed.

After over 17,000 girls underwent physical examinations at paediatricians’ offices across the US in 1997 in research spearheaded by Marcia Herman-Giddens, adjunct professor at the University of North Carolina's Gillings School of Global Public Health, some were seen starting to develop breasts as early as age 10.

The shift was seen to be even more striking in black girls, who began developing breasts, on average, at the age of 9.

Cause & Effect

The findings at the time shocked the “blindsided” medical community, according to Herman-Giddens, and prompted researchers to study the likely role of obesity, stress, and a plethora of other factors.

Subsequent studies have confirmed, in dozens of countries, that the age of puberty in girls has dropped by about three months per decade since the 1970s, with the pattern in boys similar, albeit less “extreme”.

Since that study in 1997, medical experts are yet to determine what combination of factors may be driving the puberty age decline or how to explain the stark differences based on race and sex.

There are several working theories under scrutiny.

Obesity

Obesity may play a role in the change.

A research group led by Dr Anders Juul, a paediatric endocrinologist at the University of Copenhagen, examined breast development in 1,100 girls in Copenhagen around the same time that Marcia Herman-Giddens fielded her research, writes The New York Times. His findings revealed that unlike the American children, the Danish group matched the traditional pattern described in medical textbooks: girls began developing breasts at an average age of 11 years old.

“I was interviewed quite a lot about the US puberty boom, as we called it. And I said, ‘it’s not happening in Denmark’”, Dr Juul is cited as saying by The New York Times. He suggested that earlier onset of puberty in the US was tied to a rise in childhood obesity there.

Studies showing that obesity has been linked to earlier periods in girls since the 1970s appears to support this theory.

A study carried out in the UK in 2021 found that hunger-limiting hormone leptin, released by fat cells, act on a part of the brain that regulates sexual development.

“I don’t think there’s much controversy that obesity is a major contributor to early puberty these days,” Dr Natalie Shaw, a paediatric endocrinologist at the National Institute of Environmental Health Sciences, was cited as saying.

Chemicals

In the decade since Herman-Giddens carried out her landmark study, Dr Juul also started to notice an increase in the number of referrals for early puberty in Copenhagen. Girls appeared to be developing breasts at 7 or 8 years old. In a 2009 study of nearly 1,000 school-aged girls in Copenhagen, Dr Juul’s team found that the average age of breast development had dropped by a year since his earlier study, to a little under 10.

Unlike US doctors, he did not blame obesity, as the body mass index of the Danish children in the study had not changed from that of the 1990s. Thus, an alternate theory was suggested: that chemical exposures are to blame.

According to Juul, girls with the earliest breast development in his 2009 study had the highest urine levels of phthalates. This substance is used to make plastics more durable and can be come across in everything from vinyl flooring to food packaging.

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In a review article published this April, Dr Juul and a team of researchers analysed hundreds of studies in boys and girls, testing for many different chemicals at different ages of exposure to find a link. Nevertheless, the analysis that comprised 23 studies failed to prove a definitive association between any individual chemical and the age of puberty.

Stress & Lifestyle

In the case of girls, other factors may also be involved in earlier puberty, with sexual abuse in early childhood also linked to earlier puberty onset by some researchers.

Stress and trauma could prompt earlier development, researchers claim, or diverse lifestyle factors, like a lack of physical activity.

“We are seeing these marked changes in all our children, and we don’t know how to prevent it if we wanted to”, said Dr Anders Juul, adding:

“We don’t know what is the cause”.