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SALT LAKE CITY — Girls have been getting their periods earlier and earlier for more than half a century. And as early onset increased, it has taken longer and longer for menstrual cycles to become regular. Both of those facts carry the possibility of health challenges as the girls get older.
A new study led by Harvard T.H. Chan School of Public Health researchers published this week in JAMA Network Open documented the change in average menarche among 71,341 women born between 1950 and 2005 to track the trends in menstrual cycle onset. The researchers grouped them by the decades in which they were born and again by the time it took for cycles to be regular, which used to happen within a couple of years but now often takes longer.
The trends were even stronger in girls who were Black, Asian and Hispanic and those of lower socioeconomic status.
The adverse health outcomes of early period onset cited by the researchers include cardiovascular disease, cancers, spontaneous abortion and premature death.
Some of the risk factors some believe lead to early menarche are also risk factors linked to the health challenges, such as high body mass index when young; the U.S. has a childhood obesity problem. But, according to the study, "whether obesity is the primary factor underlying the trends in menarche remains debatable. Whether and to what extent the trend in menarche is attributable to changes in early-life body mass index remains to be determined," though researchers said that's at least part of the story.
Longer time to regular cycles, which signals the maturation of the reproductive axis, is itself linked to health problems, including longer menstrual cycles and higher risk of metabolic conditions and all-cause death, the study reported.
"Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign," co-principal investigator Shruthi Mahalingaiah, assistant professor of environmental, reproductive and women's health at Harvard T.H. Chan School, said in a news release on the study.
Study nuts and bolts
The researchers used the Apple Women's Health Study, described as "a longitudinal study of menstrual cycles, gynecological conditions and overall women's health conducted by Harvard Chan School, the National Institute of Environmental Health Sciences and Apple." The study used cycle-tracking data from Apple watches and iPhones, augmented by surveys.
Researchers also got data on weight and height at time of first period from a subset of 9,865 study participants.
Health risks with early period, late regulation
The average age of menarche was 12.2 years, but close to 13% had periods start before age 11. The report found that the share of girls having periods before 11 or having very early periods before age nine were higher in the youngest age bracket groups in the study.
Between the oldest group, born between 1950 and 1969, and the youngest, born between 2000 and 2005, the number experiencing early menarche rose from 8.6% to 15.5%. And the number having late menarche dropped from 5.5% to 1.7%. As for regularity within two years — once considered the norm — the share in the older group was 76.3%, while for the younger group, the share dropped to 56%. Among those who hadn't reached regular cycles, the share in the older group had been 3.4%. Among the younger, it was 18.9%.
"We found that children are experiencing longer time to regularity," Zifan Wang, the study's lead author and a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health, told The Washington Post. "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."
Seeking the 'why'
While obesity likely accounts for some of the drop in age of menarche, it doesn't account for all of it, the study said. And most of the decrease in the average age at which girls got periods occurred before the obesity epidemic, "suggesting that other factors need to be explored to explain these trends and disparities."
The study's list of potentials includes environmental factors, such as endocrine-disrupting chemicals, metals or air pollutants, which could impact the timing of puberty. And since racial and ethnic minority groups tend to have higher exposures, that could help explain why they also have higher rates of early puberty.
The list also notes the impact of what people eat, including sugar intake. Again, those of lower socioeconomic status and even some racial groups may have eating habits that play at least some role. Researchers also list psychosocial stress and adverse childhood experiences as likely contributing at least something to the trends.
They said more study is needed into early interventions, as well as on the role of BMI and other possible factors.
Among the study's limitations was the fact that all of the information on that first period was self-report, sometimes after many years. And they didn't learn a lot about early-life factors that could contribute to the trends. They noted that the results may not be generalizable because of potential selection bias. The participants were not a random sample.
Other explorations of menarche
But the findings "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," as the Washington Post reported.
In 2021, University of California San Francisco's Department of Epidemiology & Biostatistics featured an article on possible drivers of early menarche. The article noted that in the 1890s, menarche began at 17, and by the 1950s had fallen to 12. That university's study found that "lower socioeconomic status was a significant predictor" of early puberty and menarche among Asian Americans, Black people, Hispanics and white people.
The study, led by Dr. Robert Hiatt, who regularly looks at social risk factors for breast cancer, examined the link between lower socioeconomic status and obesity, as well as poverty's ability to introduce adversity into young lives, possibly triggering the so-called "thrifty gene hypothesis." As the article explained, nature favors "survival of the gene pool over that of the individual, so when the body perceives a threat to survival, it sends a signal to hurry up and procreate."
He said that's supported by earlier research that shows girls who grow up without a father also reach puberty earlier.
"It remains for future work to disentangle socioeconomic status from another risk factor for cancer: chemical exposures. Research has shown that people in lower-income neighborhoods have higher exposures to industrial chemicals. But girls from more well-to-do families may have higher exposures to the endocrine-disrupting chemicals often found in household products and cosmetics," Hiatt said, per that article.
