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BOSTON — Girls in the United States, especially those from racial minorities and lower-income backgrounds, are starting their periods at a younger age than ever before. This trend, known as early menarche, is raising concerns among health experts. Moreover, researchers from the Harvard T.H. Chan School of Public Health say one of several factors that may be the cause of this shift is the growing problem of childhood obesity.
The findings, in a nutshell:
Menarche (pronounced meh-NAR-kee) is the medical term for a girl's first menstrual period, a significant milestone in her life that marks the beginning of her reproductive years.
Harvard researchers discovered that girls born between 2000 and 2005 got their first periods around age 11.9, compared to girls born in the 1950s and 60s who started around age 12.5. That might not seem like a big difference, but it's significant in terms of physical development. Even more concerning, about 15% of girls born after 2000 are experiencing “early menarche” (before age 11), and over 1% are having “very early menarche” (before age 9). That's nearly double the rates seen in older generations.
But that's not all. The study published in JAMA Network Open also found that girls today are taking longer to develop regular menstrual cycles. In the past, about 76% of girls had regular periods within two years of their first one. Now, only 56% do. This irregularity can make it harder for women to predict their cycles and plan their lives accordingly.
So, what's causing this concern shift? The study also found a potential link between rising rates of childhood obesity and the onset of early puberty. As body mass index (BMI) continues to rise, especially among children, researchers saw a trend connecting the epidemic to reproductive health.
How did scientists make this discovery?
To uncover these trends, researchers at Harvard's T.H. Chan School of Public Health turned to a massive treasure trove of data: the Apple Women's Health Study. This long-term study, launched in 2018, is a collaboration between Harvard, the National Institute of Environmental Health Sciences, and Apple. It uses Apple's popular menstrual cycle tracking feature to gather self-reported data from tens of thousands of women across the U.S.
For this particular study, the team looked at data from over 71,000 women who joined the study between 2018 and 2023. These participants shared personal information like their birth year, race, socioeconomic status, and most importantly, the age when they got their first period. The researchers then grouped these women by their birth decades (1950s-60s, 70s, 80s, 90s, and 2000-2005) to see how menstrual patterns changed over time.
However, the team didn't stop at first periods. About 62,000 women also reported how long it took for their cycles to become regular. Some said up to two years, others three to four, and some even longer. A smaller group of about 9,800 women provided their body mass index (BMI) at the time of their first period. BMI is a measure that uses height and weight to estimate body fat, helping identify if someone is underweight, normal weight, overweight, or obese.
By analyzing all this data, the researchers spotted clear trends: younger generations started periods earlier and took longer to establish regular cycles. These patterns were most pronounced in Black, Hispanic, Asian, and mixed-race women, as well as those from lower socioeconomic backgrounds. The findings suggest that these groups may be more vulnerable to factors that speed up puberty, such as childhood obesity, stress, poor diet, and environmental toxins.
What do researchers say?
“Continuing to investigate early menarche and its drivers is critical,” says corresponding author Zifan Wang, a postdoctoral research fellow in Harvard Chan School’s Department of Environmental Health, in a media release. “Early menarche is associated with higher risk of adverse health outcomes, such as cardiovascular disease and cancer. To address these health concerns—which our findings suggest may begin to impact more people, with disproportionate impact on already disadvantaged populations—we need much more investment in menstrual health research.”
“Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign,” explains co-principal investigator Shruthi Mahalingaiah, an assistant professor of environmental, reproductive, and women’s health at Harvard Chan School.
