Why are European girls starting puberty earlier than before?

Children are starting puberty earlier than in the past, potentially due to a combination of environmental, lifestyle, and metabolic factors – but the reasons why are still puzzling scientists.

On average, girls start puberty at age 11, while boys start at 12. But if girls enter puberty at age 7 or younger, or if boys start at age 8 or younger, they may be diagnosed with precocious puberty, which can lead to long-term health problems like obesity, diabetes, mental health issues, and breast cancer.

When doctors first identified the shift toward earlier puberty among American girls in the 1990s, they didn’t see the same trend among European children. Researchers chalked it up to the US obesity epidemic, which wasn’t affecting Denmark, where most of the European data came from.

But from the early 2000s onwards, something changed. From Denmark to Italy and beyond, more European kids have begun starting puberty early.

Globally, the age when girls begin developing breasts – a key puberty marker – has fallen by about three months per decade between 1977 and 2013, and the trend toward early puberty doesn’t appear to have slowed in the past decade.

“As a doctor and researcher, I am concerned that we simply don’t know what is going on,” Dr Anders Juul, paediatric endocrinologist at Copenhagen University Hospital and a leading expert on puberty changes worldwide, told Euronews Health.

“If we cannot identify the worst sinners and have an explanation of why this is ongoing, we will have no way in the future to take preventive initiatives”.

Despite the uncertainties, Juul and others are beginning to find some answers. Here’s what the latest research says on the potential drivers of early puberty in Europe, and where the most pressing questions remain.

Obesity

Researchers still believe nutrition and obesity could contribute to earlier puberty because excess fat is tied to increased levels of leptin, a hormone that signals that the body is ready to start puberty.

That process affects girls and boys differently, which could help explain why puberty tends to start earlier in obese girls and later in obese boys.

Today, about one in three children in the World Health Organization’s (WHO) Europe region are overweight or obese, and rates tend to be higher in southern Europe.

However, in the early Danish studies conducted 15 years apart, there was not a significant difference in body mass index (BMI) between the first and second group of young girls, who started puberty a full year earlier on average.

“We don’t think that the increase in BMI can explain everything,” Ingvild Halsør Forthun, a PhD researcher at the University of Bergen in Norway, told Euronews Health.

Chemical exposures

One leading theory is that exposure to endocrine-disrupting chemicals found in food and everyday products could trigger earlier puberty.

In the global study on girls’ puberty timing in recent decades, Juul’s team suggested that a “cocktail” of many different chemical substances may mimic hormones and send children into puberty at a younger age.

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But more recently, the evidence on chemical factors appears inconclusive and “the jury’s still out” on the role of these compounds, Juul said.

“When we group kids according to chemical [levels], we do see striking differences,” Juul said. “When it comes to endocrine-disrupting chemicals, I think it’s a very good candidate, but we find it hard to provide solid evidence”.

Meanwhile, a recent Norwegian study found that exposure to per- and polyfluoroalkyl substances (PFAS) – known as “forever chemicals” because they are ubiquitous in consumer products and take hundreds of years to break down – could affect puberty timing in boys.

Notably, however, it showed that boys with higher PFAS concentrations were actually starting puberty later than their peers, shedding light on how boys and girls are affected differently by chemical exposures, and underscoring that these compounds trigger the body in different ways.

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“But the mechanisms behind it, we don’t know for sure,” said Halsør Forthun, one of the study authors.

Her team measured puberty markers like testicular volume, pubic hair onset, and serum levels of testosterone and other hormones – a more objective approach, she said, than the self-reported data often used in puberty-timing studies.

Going forward, Halsør Forthun will be using similar methods to analyse PFAS exposure and puberty timing in Norwegian girls.

Lifestyle factors

During the early months of the COVID-19 pandemic, Italian doctors noticed a concerning trend: more and more young girls were turning up to paediatric endocrinology clinics with signs of precocious puberty, such as early breast tissue development.

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From March to September 2020, there were 328 referrals for girls, up from 140 in the same period the year before. But then the referrals dropped off again in 2022.

Researchers believe the increase in sedentary lifestyles and phone use during the early pandemic era may be tied to the uptick.

Physical activity “declined dramatically during the lockdown period, and that increased again during the recovery. The same for weekly screen time,” Dr Marco Cappa, a paediatric endocrinologist at Bambino Gesù Children’s Hospital in Rome and one of the study’s authors, told Euronews Health.

“But the interesting thing is that [there was] no change in BMI,” he added, meaning physical activity may be separately associated with puberty onset.

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Even so, Cappa also acknowledged that intense stress during the pandemic could also have driven the changes they saw.

Other potential drivers

Researchers have explored whether stress – both during pregnancy and a child’s life – could explain early puberty. Kids from families with absent fathers, for example, report higher levels of early puberty regardless of their gender.

Other family factors may play a role too. Only children tend to enter puberty earlier than kids with biological siblings, according to a 10,700-person study from Aarhus University in Denmark.

Several of these factors could overlap, researchers said, but it’s still not clear whether these factors are actually causing early puberty, or if they’re just happening in tandem.

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For example, mothers transfer PFAS to their babies during pregnancy and when they breastfeed, and firstborn children tend to have higher PFAS levels than their younger siblings, Halsør Forthun said.

Further, children from lower-income households are more likely to be obese than their wealthier peers, which could point to a combination of socioeconomic and metabolic factors.