Menstrual Cycles Are Starting Earlier in Younger Generations, Harvard Study Finds

by Ella

A new study from Harvard University, published in JAMA Network Open, reveals that girls are starting their menstrual cycles earlier, raising important health implications across generations.


Earlier Menarche and Health Risks

The onset of menstruation, known as menarche, has been linked to various adverse health outcomes, including an increased risk of cardiovascular disease and cancer. Typically, it takes one to two years for menstrual cycles to become regular, with delays in this regularity associated with metabolic conditions and other health risks.


Previous research has documented a decrease in the average age of menarche over the last 50 years. However, there has been limited exploration of how race and socioeconomic factors influence this trend, and few studies have examined changes in the time it takes for menstrual cycles to become regular.


“Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign,” said Dr. Shruthi Mahalingaiah, assistant professor of Environmental, Reproductive and Women’s Health at Harvard Chan School.


Study Insights and Data

The researchers utilized data from the Apple Women’s Health Study, a longitudinal investigation into menstrual cycles, gynecological conditions, and women’s overall health. The study enrolled over 71,000 participants between November 2018 and March 2023. Participants reported the age at which they had their first menstrual cycle, along with their race and socioeconomic status. They were categorized into five age brackets based on their birth years.

The age of menarche was defined as:

Early: younger than 11 years old

Very early: younger than 9 years old

Late: aged 16 and above

A subset of 61,932 participants provided information on the time it took for their cycles to become regular, and 9,865 participants provided their body mass index (BMI) at the time of their first cycle.

Trends Among Racial Minorities

The study found that the average age at menarche has decreased over time. Participants born between 1950-1969 had an average menarche age of 12.5 years, compared to those born between 2000-2005, who had an average age of 11.9 years. The rates of early and very early menarche nearly doubled from 8.6% and 0.6% in the 1950-1969 group to 15.5% and 1.4% in the 2000-2005 group, respectively. Additionally, the percentage of participants achieving regular menstrual cycles within two years decreased from 76% to 56%.

These trends were evident across all socioeconomic groups but were more pronounced among participants identifying as Black, Hispanic, Asian, or mixed race, and those from lower socioeconomic backgrounds.

“Transethnic genome-wide association studies indicated that these disparities are unlikely to be attributed to genetic variations, suggesting they may be driven by other environmental or contextual factors,” the authors wrote.

Impact of BMI on Menarche

Childhood obesity, a growing epidemic in the US and other countries, is a known risk factor for early puberty. The researchers suggest that 46% of the temporal trends in menarche can be explained by an increase in BMI. However, BMI does not account for the remaining 54%. Previous research has shown that the largest decrease in the age of menarche occurred before the obesity crisis in the US. Further studies are needed to explore other contributing factors, such as exposure to endocrine-disrupting chemicals, dietary patterns, and psychological stress.

Expert Commentary

Dr. Michelle Wise, an associate professor in the Department of Obstetrics and Gynaecology at the University of Auckland, highlighted the practical implications of the study: “In my practice, we see many women with irregular menstrual cycles, heavy menstrual bleeding leading to anemia and poor quality of life, new diagnoses of endometrial cancer in women in their 30s and 40s, and couples having difficulty conceiving. The epidemic of obesity is contributing to this, and not enough is being done at the societal and policy level to address it.”

“Continuing to investigate early menarche and its drivers is critical,” said lead author Dr. Zifan Wang, postdoctoral research fellow at Harvard Chan School’s Department of Environmental Health. “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.”

This study underscores the need for a deeper understanding of menstrual health and its long-term implications, highlighting the importance of addressing environmental and socioeconomic factors to improve women’s health outcomes.


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