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, Dr. Herman-Giddens recalled. “They were blindsided,” she said…

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.