Why are girls starting puberty younger?

During the Covid pandemic, doctors began to notice a curious phenomenon: the numbers of young girls experiencing puberty early were notably increasing. It was happening in multiple countries, with similar reports coming from Italy, Turkey, the US, India and Germany. And it didn’t seem as if the virus itself was to blame, as the rise in cases was not restricted to females who had been infected with Covid.

While the age at which girls enter puberty had been steadily declining by about three months every 10 years since the late 1970s, something different appeared to happen once the world was in the grip of the pandemic. Paediatricians witnessed a spike in what is known as precocious puberty, which went beyond this broad, long-term trend. Several studies have since been carried out, all indicating a correlation between the pandemic and premature menstruation and sexual development.

It is normal for girls to start puberty between eight and 13, according to the NHS. However, if it starts before the age of eight, it is termed precocious puberty. Pre-pandemic, global stats showed only one in every 5,000 to 10,000 children went through puberty before age eight. But for reasons that are still unclear, the pandemic saw this figure start to climb - in girls but not in boys (who were already far less likely to enter puberty early).

An Italian study published in February last year, which looked at five paediatric endocrinology centres, revealed that between March and September 2020, 338 children had been referred for suspected precocious puberty. This compared to 152 children during the same period of 2019, representing an increase of 122 per cent.

A recent study, based on a Turkish endocrinology clinic and published last August, found more than twice as many girls were diagnosed with precocious puberty between April 2020 and March 2021 than during the two years before the start of the pandemic. And a German study, based on one medical centre, discovered the number of girls diagnosed with early puberty had more than doubled in the first year of the pandemic to 23, compared to fewer than 10 cases a year between 2015 and 2019. In 2021, the number rose to 30. Last September, the findings of several studies on increased incidence of early puberty were presented to the annual meeting of European Society for Paediatric Endocrinology.

Unclear causes

So what is going on? If the pandemic was to blame, then why so? Experts stress there are no proven causes yet. “Right now it’s not possible to directly link the ‘trauma’ caused by the pandemic to the number of cases,” says Dr Marie-Agathe Trouvin, clinical director at the Hôpitaux de Paris.

But several explanations have been suggested. Previous studies have shown overweight or obese children are statistically more likely to go through puberty early, and indeed the long-term decline in the average age of puberty has broadly coincided with the rise in childhood obesity.

Prolonged screen time has also been cited as a potential factor, with a Turkish study of rats suggesting an association between higher levels of blue light exposure and earlier onset of puberty for females. Since the pandemic saw many children’s lifestyles become more sedentary and screen-based, with opportunities for exercise diminishing and obesity increasing further, could this explain why clinicians were seeing so many more cases of precocious puberty? Some of those who have carried out previous research in this area believe it’s entirely plausible.

Long-term trend

In 2016, researchers conducted the largest UK study on early puberty to date, defining “early” as kicking in before age 11. Drawing on a sample of almost 6,000 girls from the comprehensive Millennium Cohort Study, which follows the lives of young people born between 2000 and 2002, they found about 10 per cent had begun menstruation at 10 or younger. The average age was 12-and-a-half but, says Professor Yvonne Kelly from University College London, who co-authored the research, “we know age of puberty has been coming down for decades and decades.”

It had long been hypothesised that the age of puberty was declining for three reasons: higher levels of obesity in the population, greater stress levels and environmental toxins. Prof Kelly and her fellow researchers tested the first two theories (they did not have data on pollution) and found that both did appear to be contributing to early menstruation in girls.

“If we think about the pandemic, it’s no surprise [if more children entered puberty early], given what we’ve seen in the UK,” she says. “We know there’s been a massive spike in obesity rates in Britain in 2020 and 2021. And we’re seeing soaring rates of poor mental health and children living in stressful circumstances.” Weight gain, then, became an initial suspect. But its culpability hasn’t been proven.

“Our first hypothesis was that the weight gain during lockdown could affect the onset of puberty,” says Dr Trouvin. “But we didn’t find any link in our patients between increased body mass index and puberty onset compared to [during the] years before Covid-19.”

The role of stress

Stress is another suspect. Professor Ruben Gur, director of the Brain Behaviour Laboratory at the University of Pennsylvania, studied a cohort of 10,000 young people from the Greater Philadelphia area between 2009 and 2012 and found those exposed to trauma achieved physical maturity faster. His study also revealed the brains of these children developed faster.

“Covid-19 [was] a traumatic event for everyone and, depending on how much stress was associated with it, it makes sense those kids will show the same signs of early maturation,” he says, “I’m not surprised [by the increased number going through early puberty]. It’s totally in line with what we found.”

Prof Kelly says she would not be surprised if the spike in precocious puberty cases seen elsewhere also turned out to have occurred in the UK. Would it matter if so? Research on the effects of early puberty suggests it would be cause for concern. A University of Bristol study in 2020 found both girls and boys who experience puberty earlier than their peers have an increased risk of self-harm in adolescence. Other research shows they are at a higher risk of mental health problems such as depression.

“Once their hormones start kicking in… it’s a really confusing time in young women’s lives,” says Prof Kelly. “The earlier that happens, young people could be less well equipped to deal with the psychological consequences.”

It can also be socially challenging for young girls. And it can have adverse physical effects. Various studies link early puberty with poorer physical health in later life, including a greater risk of early onset heart disease. Additionally, it has implications for growth.

“Once puberty begins in terms of menarche [the first menstrual period], height growth slows,” says Prof Kelly. “So, if a girl is seven or eight, she’ll no way be of adult stature. After [a girl’s] period starts, they only grow about three or four more inches.”

The standard treatment for precocious puberty is puberty blockers - medications that block the hormones coming from the pituitary gland that trigger it. Not everyone agrees, however, with the narrative that precocious puberty is rising - or that we should be medicating it.

“The fact that more children have presented with early puberty, does not mean it is on the increase,” argues Dr Peter Hayes, a senior lecturer in politics at the University of Sunderland and author of a 2016 article, Early Puberty, Medicalisation and the Ideology of Normality. “There are any number of reasons why the rate may have remained the same, but the reporting rate increased.”

He cites the possibility that during the pandemic parents experienced heightened anxiety about their children's health and were more likely to report signs of early puberty. Or, he suggests, parents spent more time in physical proximity with their children during lockdown, and so were perhaps more likely to notice bodily and behavioural changes.

“Don't be too alarmed by the long list of illnesses supposedly linked to early puberty,” he adds. “Doctors are always trying to find such problems in order to justify medicating children who are perfectly healthy but just happen to be maturing early.”