Menstruation is a critical indicator of health. Whether and when someone with a uterus gets their period — for the first time, and throughout their life — can reflect not only their reproductive health, but their risk of cardiovascular disease, cancer, miscarriage, and premature death.
That also makes menstruation a useful measure of population health. And digital tools for clinical research are beginning to shed light on just how significantly periods are changing over time.
A study published Wednesday in JAMA Network Open, based on data from more than 71,000 women collected through the Apple Research App, shows that girls in the United States have been getting their first period earlier and earlier over the last 55 years — and it has taken longer for their periods to become regular, pointing to worsening overall reproductive and population health.
The research is part of the large Apple Women’s Health Study, launched in 2019 and conducted with the National Institutes of Health and Harvard T.H. Chan School of Public Health. They found the average age at menarche — when an individual gets their first period — decreased from 12.5 years in 1950 to 1969 to 11.9 years in 2000 to 2005. In the same period, the percentage of people who experienced early menarche (before age 11) increased from 8.6% to 15.5%, and the percentage of those having very early menarche (before age 9) more than doubled from 0.6% to 1.4%.
Among the 62,000 participants who shared data on cycle regularity, the percentage whose cycles became regular within two years of menarche decreased by 74% to 56% between 1950-1969 and 2000-2005.
Both early menarche and irregular cycles are associated with health issues. An early first period is associated with increased risk of cardiovascular disease, cancer, and early death, typically connected to increased exposure to estrogen. And a longer window to reach cycle stability in the early years is associated with increased chances of irregular cycles during the rest of reproductive life, which can be a cause of infertility.
It’s not clear exactly why the experience of menstruation is changing so much. “The determinants of menarche, and the pubertal transition, are multifactorial,” wrote study co-author Shruthi Mahalingaiah, an assistant professor of environmental, reproductive, and women’s health at Harvard, in an email to STAT. They can be related to body composition, diet, physical activity, and stress, along with environmental exposure to endocrine-disrupting chemicals and air pollutants.
But the study offers clues. About 10,000 participants provided data on their body mass index, and the analysis estimated that about 46% of the drop in age of menarche could be explained by higher BMI, in part because puberty is dependent on accumulation of sufficient body fat.
“I think that that is very reflective of the obesity epidemic that we are seeing,” said Aviva Sopher, a professor of pediatrics at Columbia University, Irving Medical Center, who was not involved in the study. She suggested the changes could be related to a possible increase in polycystic ovary syndrome. “To me, this is concerning as well, this kind of confluence of obesity and earlier puberty and more irregular cycles,” she said.
Obesity may not tell the whole story. “I think obesity is one factor. I don’t think it explained the trends fully,” said Lauren Houghton, an epidemiologist at the Columbia University Mailman School of Public Health and the author of a commentary accompanying the study, also published in JAMA Network Open. This study used a one-time measurement of BMI that was close to the age of menarche; to validate the connection, she said, researchers would need to conduct prospective longitudinal studies on age of menarche and body mass.
“Another concern is stress and the effect of stress on earlier puberty and earlier menarche,” said Houghton, who highlighted that during Covid-19, stress was associated with younger age at puberty. Stress could also be driven by environmental factors like financial instability or racism: In the study, racial and ethnic minority groups and people of lower socioeconomic status were most likely to experience cycle irregularity and a reduction in the age of first menstruation.
Early menarche could also be a cause of further stress down the line, said Sopher, who has researched the psychological consequences of early menstruation. “With more girls, going through puberty earlier, it might make it more comfortable for those experiencing it,” she said. “But on the other hand, in general, girls who go through puberty earlier are at increased risk of sexual abuse because they look very mature, but they’re immature.”
Overall, the research stands as an important reminder of the value of menstruation data as a health indicator, said Houghton. “For most of history, people have thought of menstruation as noise or nuisance,” she said. But as individuals have turned to wearables and mobile apps to track their cycles more regularly, menstruation empowers people to better understand their own unique cycle, she said.
Someone who has a consistently short cycle may want to bring a change in length to the attention of a medical provider, for example, even if it still appears as a “normal” 28-day cycle. For providers, the data offers important information on a person’s health, including potential reproductive and endocrine issues.
And for population-level health researchers, large-scale data like that from the Apple Women’s Health Study offer the opportunity to understand factors affecting menstrual and fertility trends — including the reproductive health of gender-diverse populations, said Houghton — and what kind of systemic interventions are necessary to improve it.
Menstrual variation could be used to enhance diagnostics and provide a better picture of individual and population health, for example. “We know that biomarkers for cardiovascular disease differ depending on where you are in your menstrual cycle, and then it can lead to misdiagnoses and underdiagnoses,” said Houghton; mammogram quality varies by menstrual stage, too.
Menstrual health and women’s health is understudied and underfunded. “We need significantly greater investment in early counseling, education around menstrual health as a vital sign, and individualized healthcare plans,” wrote Mahalingaiah.
“We are making strides right now … but it doesn’t make up for like decades of being understudied,” Houghton added. “And so for me, the ability to use the app data is amazing. It makes me be able to do research that I was never able to do before — and with no funding.”