Girls’ periods are starting sooner, more irregular than past generations

The data were collected as part of the Apple Women’s Health Study, which was developed by the Harvard T.H. Chan School of Public Health with the National Institute of Environmental Health Sciences (NIEHS) and Apple. The study used cycle tracking data from iPhones and Apple Watches as well as surveys to learn more about menstrual cycles, health risks and gynecologic conditions.

The findings aren’t generalizable to the overall U.S. population because the data were collected from app users recruited on social media rather than a random sample. And the study noted the data overrepresented White women and those with higher socioeconomic status.

But the findings, published in JAMA Network Open, mirror other research, including a 50-year look at menstruation age among White and Black women based on data from the National Health and Nutrition Examination Survey, a government report on health trends.

The new JAMA study showed that the average age of first menstruation in the Apple cohort dropped to 11.9 years among those born between 2000 and 2005, compared with 12.5 years for those born between 1950 and 1969. Participants who self-identified as Asian, non-Hispanic Black or multiracial consistently reported earlier average ages of first periods than participants who were White.

Researchers said the data are important because Hispanic and Asian groups have been understudied in previous research looking at age of first period. The study also focused on what it called a new “vital sign” — the time between first period and regular menstrual cycles.

“We found that children are experiencing longer time to regularity,” said Zifan Wang, the study’s lead author and a post doctoral research fellow at the Harvard T.H. Chan School of Public Health. “This is also very concerning because irregular cycles are an important indicator of later-in-life adverse health events. It alarms us. We need to do more early counseling and intervention on irregular cycles among children and adolescents.”

The data also showed that the proportion of girls in the cohort who begin having periods before the age of 11, or very early periods, before the age of 9, was higher in the latest birth year group compared with the earliest group.

Girls who start periods at very young ages face more challenging health issues later in life, said Shruthi Mahalingaiah, the study’s senior author and assistant professor of environmental, reproductive and women’s health at the Harvard School of Public Health. Mahalingaiah said that early periods can be a marker for future health conditions and help physicians make decisions about care. She noted that a healthy diet, exercise and adequate sleep are important for girls of all ages.

“I’m going to point back to awareness and education so parents and providers are aware of these trends,” she said. “We need to consider the health-promoting factors that we can implement to impact not only age of menarche but time to cycle regularity.”

What else you should know:

Researchers offered several explanations for the trend toward earlier periods. Childhood obesity is a risk factor for early puberty and appears to be a contributing factor in the trends. But the decrease in age of first period began before the obesity epidemic, suggesting other factors are at play.

One major concern is “forever chemicals,” a class of nearly 15,000 man-made chemicals used in a variety of consumer products and found in many food and water sources. These endocrine-disrupting chemicals, as well as heavy metals and air pollutants, all may play a role in early puberty.

And importantly, poor diet including high intake of sugary foods, stress and adverse childhood experience can also affect onset of puberty. Some studies have shown that very early puberty rose during the pandemic, possibly because of the stress and disruption it caused in the lives of young children.

Research suggests that in general, girls are getting their periods earlier than previous generations, and cycle irregularity also appears to be on the rise.