Puberty starts earlier than it used to, and no one knows why

Marcia Herman-Giddens first realized something was changing in young girls in the late 1980s, while she was serving as director for the child abuse team at Duke University Medical Center in Durham, North Carolina. During evaluations of girls who had been abused, Herman-Giddens noticed that many of them had started developing breasts as young as 6 or 7.

“That did not seem right,” said Herman-Giddens, who is now an adjunct professor at the University of North Carolina Gillings School of Global Public Health. She wondered whether girls with early breast development were more likely to be sexually abused, but she could not find any data keeping track of puberty onset in girls in the United States. So she decided to collect it herself.

A decade later, she published a study of more than 17,000 girls who underwent physical examinations at pediatricians’ offices across the country. The numbers revealed that, on average, girls in the mid-1990s had started to develop breasts — typically the first sign of puberty — around age 10, more than a year earlier than previously recorded. The decline was even more striking in Black girls, who had begun developing breasts, on average, at age 9.

The medical community was shocked by the findings, and many were doubtful about a dramatic new trend spotted by an unknown physician assistant, Herman-Giddens recalled. “They were blindsided,” she said.

But the study turned out to be a watershed in the medical understanding of puberty. Studies in the decades since have confirmed, in dozens of countries, that the age of puberty in girls has dropped by about three months per decade since the 1970s. A similar pattern, though less extreme, has been observed in boys.

Although it is difficult to tease apart cause and effect, earlier puberty may have harmful impacts, especially for girls. Girls who go through puberty early are at a higher risk of depression, anxiety, substance abuse and other psychological problems, compared with peers who hit puberty later. Girls who get their periods earlier may also be at a higher risk of developing breast or uterine cancer in adulthood.

No one knows what risk factor — or more likely, what combination of factors — is driving the age decline or why there are stark race- and sex-based differences. Obesity seems to be playing a role, but it cannot fully explain the change. Researchers are also investigating other potential influences, including chemicals found in certain plastics and stress. And for unclear reasons, doctors across the world have reported a rise in early puberty cases during the pandemic.

“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, a pediatric endocrinologist at the University of Copenhagen who has published two recent studies on the phenomenon. “We don’t know what is the cause.”

Obesity

Around the time Herman-Giddens published her landmark study, Juul’s research group examined breast development in a cohort of 1,100 girls in Copenhagen, Denmark. Unlike the American children, the Danish group matched the pattern long described in medical textbooks: Girls began developing breasts at an average age of 11 years old.

“I was interviewed quite a lot about the U.S. puberty boom, as we called it,” Juul said. “And I said, ‘it’s not happening in Denmark.’”

At the time, Juul suggested that the earlier onset of puberty in the United States was probably tied to a rise in childhood obesity, which had not occurred in Denmark.

Obesity has been linked to earlier periods in girls since the 1970s. Numerous studies since have established that girls who are overweight or obese tend to start their periods earlier than girls of average weights do.

“I don’t think there’s much controversy that obesity is a major contributor to early puberty these days,” said Dr. Natalie Shaw, a pediatric endocrinologist at the National Institute of Environmental Health Sciences who has studied the effects of obesity on puberty.

Still, she added, many girls who develop early are