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The covid-19 pandemic may be triggering early puberty in some girls. Several studies suggest the outbreak is increasing the number of girls going through premature sexual development – and experts are unsure why.

In the latest of a string of studies, researchers at the University of Bonn, Germany, reported how the number of girls diagnosed with early puberty at a single medical centre remained constant between 2015 and 2019, at fewer than 10 cases a year.

This more than doubled to 23 in 2020, when the covid-19 outbreak took hold worldwide, rising further still to 30 in 2021, according to results presented at The European Society for Paediatric Endocrinology 2022 meeting today.

The German researchers aren’t the only ones to see cases double. “In the pre-covid year, we had 28 children start treatment and in the covid year, we had 64 children start treatment,” says Karen Klein at Rady Children’s Hospital and the University of California, San Diego.

Similar results have also been reported in and .

Early puberty is rare, in pre-pandemic times. For every 1 boy, it affects 10 girls. The reasons behind this sexual disparity are unclear.

Regardless of a person’s sex, early puberty is linked to short stature in adulthood, . Early puberty has also been , such as anxiety in boys and depression in girls.

Sezer Acar at Dr. Behçet Uz Children’s Education and Research Hospital in Izmir, an author of the Turkish study, says: “Previously, I [treated] one or two patients a month due to precocious [early] puberty, but during this period [the early stages of the pandemic before his study was published], I had to treat two or three patients a week.”

In addition to an increased number of girls starting puberty early, the age of onset may have also declined.

In the German study, pre-pandemic puberty onset occurred at age 7.6, on average, compared with 6.8 among those diagnosed during the covid-19 outbreak. A statistical analysis suggests this wasn’t a chance finding.

“We know stress can cause earlier puberty, so that’s certainly high on the list of what’s going on,” says Klein.

“The other thing people immediately started to think about was, well, everyone is at home not exercising as much and maybe it’s weight gain, because we know rapid weight gain can cause earlier puberty. But in our study and in a couple of other studies, we didn’t see that the children were heavier.”

Increased screen time and changes in sleep cycles due to remote learning could also be at play, says Paul Kaplowitz at the Children’s National Hospital in Washington D.C.

These factors weren’t assessed in all the studies. However, , researchers found that girls who were diagnosed with early puberty during lockdown had more disturbed sleep and later bedtimes than those diagnosed pre-pandemic.

Some have questioned whether SARS-CoV-2 itself could be to blame. Inflammation of the nasal cavity has been and people going through Although this hypothesis can’t be ruled out, especially because many childhood covid-19 cases are mild and may be missed, it seems unlikely, says Kaplowitz.

“I don’t think the effect of covid on female puberty is restricted to girls who actually had the infection,” says Kaplowitz. “Especially since, in the earlier stages of the pandemic, children were much less likely to become infected than adults.”

The pandemic aside, the age of puberty onset has been since 1977, although there is little data on the effect of other traumatic events like wars or recessions.

Medication can reduce hormone levels and stunt sexual development for several years. However, .

Some doctors hope that the return of in-person schooling and children adapting to pandemic-related challenges will slow the rate of early puberty.

“When the data is looked at for the past year, particularly in places where kids almost all went back to school and life returned to more normal, I would predict that the rate of precocious puberty will return to what it had been previously,” says Kaplowitz. “But we obviously don’t know.”