A new study from the Harvard T.H. Chan School of Public Health, the National Institute of Environmental Health Sciences, and Apple has revealed that the average age at which girls in the United States begin menstruating has been decreasing over the past several decades.
The study, which analyzed data from more than 71,000 participants in the Apple Women’s Health Study, also found that the time it takes for menstrual cycles to become regular is increasing, with these trends being most pronounced among racial minorities and those from lower socioeconomic backgrounds.
The study, published in JAMA Network Open, is one of the first to examine menstrual cycle trends in the context of racial and socioeconomic differences, as well as one of the first to investigate trends in the time it takes for menstrual cycles to become regular. According to Shruthi Mahalingaiah, co-principal investigator and assistant professor at Harvard Chan School, “Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign.”
Younger Generations Experiencing Earlier Menarche and Longer Time to Regularity
The researchers divided the study participants into five age brackets based on their birth years, ranging from 1950-1969 to 2000-2005. They found that as birth year increased, the average age at menarche decreased, and the time from menarche to menstrual cycle regularity increased. Among participants born between 1950-1969, the average age at menarche was 12.5 years, with 8.6% experiencing early menarche (younger than 11 years old) and 0.6% experiencing very early menarche (younger than 9 years old). In contrast, among participants born between 2000-2005, the average age at menarche was 11.9 years, with 15.5% experiencing early menarche and 1.4% experiencing very early menarche.
The study also found that the percentage of participants who reached menstrual cycle regularity within two years of menarche decreased from 76% among those born between 1950-1969 to 56% among those born between 2000-2005. These trends were present across all sociodemographic groups but were most pronounced among participants who identified as Black, Hispanic, Asian, or mixed race, and those who rated themselves as belonging to a low socioeconomic status.
Potential Factors Contributing to Earlier Menarche and Longer Time to Regularity
The study’s findings suggest that childhood obesity, a growing epidemic in the U.S. and a known risk factor for early puberty, could be a contributing factor to earlier menarche. Other possible factors include dietary patterns, psychological stress and adverse childhood experiences, and environmental factors such as endocrine-disrupting chemicals and air pollution.
Zifan Wang, postdoctoral research fellow at Harvard Chan School and corresponding author of the study, emphasized the importance of continuing to investigate early menarche and its drivers, stating, “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.”
While the study relied heavily on retrospective self-reporting, its large and diverse dataset from the Apple Women’s Health Study allowed researchers to fill important gaps in understanding menstrual health trends across different racial and socioeconomic groups in the United States. The findings underscore the need for further research and investment in this critical area of women’s health.
“Menarche and time to cycle regularity among females born between 1950-2005 in the US,” Zifan Wang, Gowtham Asokan, Jukka-Pekka Onnela, Donna D. Baird, Anne Marie Z. Jukic, Allen J. Wilcox, Christine L. Curry, Tyler Fischer-Colbrie, Michelle A. Williams, Russ Hauser, Brent A. Coull, Shruthi Mahalingaiah, JAMA Network Open, May 29, 2024, doi: 10.1001/jamanetworkopen.2024.12854
