Why are more and more girls starting puberty early?

Shortly after the start of the COVID pandemic, doctors in endocrinology clinics across the world started noticing something peculiar. More and more girls were showing up with cases of precocious puberty, a rare condition characterized by the emergence of signs of puberty before turning 7 or 8.

On average, girls start puberty between the ages of 8 and 13. If their bodies begin to develop before then, doctors consider it precocious puberty — a very uncommon condition that, before the pandemic, researchers estimated only impacted between 5,000 and 10,000 children across the globe.

But since COVID, cases seem to be increasing everywhere. At one endocrinology clinic in Italy, the number of patients arriving with cases of suspected precocious puberty grew from 118 in 2019 to 246 in 2020, a 108% increase. In another Italian clinic, doctors reported seeing 90 cases during the COVID outbreak compared with just 64 in the previous three years.

In a Turkish clinic, doctors reported that the number of children referred for precocious puberty during the pandemic had more than doubled, from an average of 20 per year to 58 between March 2020 and April 2021.

In a clinic in India, doctors reported that the number of girls presenting with precocious puberty had increased from 54 before the lockdown to 146 during it. And in Shanghai, researchers saw an increase from an average of 106 patients in the years between 2016 and 2019 to 372 in 2020.

Age of menstruation declining

Doctors have known for decades that the age girls start puberty has been declining since the start of the 20th century. The age of menstruation declined from around 16 in the mid-19th century to around 13 in the 1980s. Since then, however, that age has only declined slightly, hovering somewhere between 12 and 13 years old in most parts of the world.

More noticeable, researchers say, has been a decline in the age that girls start developing breast tissue, which is generally considered the start of puberty and occurs around two years before menstruation. According to a large Danish study published in 2009, the age at which young girls develop breast tissue had declined by around one year between 1991 and 2006. Authors say this could be an indication that the puberty process is starting earlier and lasting longer than in the past.

What's causing early puberty

Before COVID, puberty researchers had formed three hypotheses about what could be causing the rise in early development: Obesity, stress and pesticides. Each of these ideas has worked to explain the rise in puberty levels in certain environments, but none account for the phenomenon as a whole. 

In general, researchers have attributed the uptick since the start of the pandemic to stress, rather than a potential reponse to COVID infection or vaccination or higher rates of obesity among young girls. 

Many studies have shown a higher rate of early puberty in children who are overweight or obese than in children at normal weights. One study evaluating more than 17,000 children in China found that  nearly 40% of boys and 30% of girls with precocious puberty were obese, while a systematic review of 10 studies, published this summer in Frontiers, found that children, and especially girls, who were obese or overweight were at a "significantly higher risk" of early puberty than children at normal weights.

But not all children who experience precocious puberty, or merely develop earlier than their peers, are obese or overweight. For example: In the Turkish clinic where cases of precocious puberty more than doubled to 58 children during the pandemic, just 9 were obese.

"The current study reported no significant increase in BMI compared to the pre-pandemic period, suggesting that factors other than increased BMI may play a role in the development of [precocious puberty]," the authors wrote.

The study suggested that some of those factors could include the impact that increased use of digital devices has on sleep, as well as psychological factors like stress induced by lockdowns and sick family members.

Past studies have shown a positive correlation between stress and the early onset of puberty: A paper published in BMC Pediatrics suggests that girls who don't live with both parents as babies may be at a higher risk of early puberty than girls who grew up in households with both parents.

"Unusual timing of puberty, usually early, is linked to stressful life experiences," said Jane Mendle, a psychology professor at Cornell University who studies puberty. "And I cannot think of anything more stressful and destabilizing than the COVID-19 pandemic, particularly in those early intense days on lockdown."

Psychological impact 

In the United States, studies show that early puberty is associated with higher rates of depression. This is true for both boys and girls, Karen Rudolph, a professor who studies puberty at the University of Illinois, said. She added that for girls, those high rates of depression are likely to remain into adulthood. 

That's not necessarily the case for boys, who are more likely to experience higher rates of depression if they develop later, she said. Studies show that the depression slows when these boys catch up to their peers. 

Rudolph stressed that it is not the case that every girl who experiences early puberty will become depressed. 

"It's certainly not deterministic," she said.

Aspects that can predict whether an early bloomer might be at higher risk of developing depression include high levels of stress in peer groups or the family, as well as whether the child has a parent with depression. Rudolph added that girls with fewer resources for dealing with stress are also at higher risk.

She said her research indicates girls with mothers who teach them how to cope with stressors are less likely to develop depression after hitting puberty early. 

"These girls are not any more likely to be depressed than girls who go through puberty later. But when they have mothers who don't do much of that kind of teaching, they're more likely to be depressed," Rudolph said. 

Cultural aspect

The bulk of studies that have investigated the psychological impact of early puberty on children have been conducted using samples of largely white children in industrialized Western countries. 

Studies facilitated in the US and Europe have shown that girls who are part of minority populations, like African American or Latina girls in the US, are more likely to start puberty early. 

But little research has been conducted outside Western industrialized countries on rates of depression in children who start puberty early. 

"We have some studies, but not as much knowledge as I would like, on how race, ethnicity, social class, intersect with the process of maturation," said Mendle. 

Edited by: Carla Bleiker