Menstrual Cycles Are Starting Earlier in Younger Generations

A new study from Harvard University investigated trends in the timing of when girls start their first period. The paper, published in JAMA Network Open, raises implications for women’s health across generations.

Menstrual periods are arriving earlier

The age at which an individual starts their first menstrual period, also referred to as menarche, has been linked with various adverse health outcomes. Early menarche is associated with a risk of cardiovascular disease and cancer, among other diseases. The menstrual cycle typically takes one–two years to establish regularity. An increased time to regularity is also linked to increased health risks, such as metabolic conditions.

Previous studies have identified a decrease in mean age at menarche in younger generations over the last five decades. However, research exploring how race and socioeconomic factors may contribute to this trend is lacking, and only a few studies have investigated changes in trends in the time to menstrual cycle regularity.

“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 corresponding author, Dr. Shruthi Mahalingaiah, assistant professor of Environmental, Reproductive and Women’s Health at Harvard Chan School.

Understanding menarche impacts

The researchers used data from the Apple Women’s Health Study, a longitudinal study investigating menstrual cycles, gynecological conditions and women’s overall health. Over 71,000 participants were enrolled between November 2018 and March 2023. The participants self-reported the age at which they had their first menstrual cycle, their race and their socioeconomic status. Participants were then divided into one of five age brackets depending on their date of birth.

Age of menarche was characterized as:

• Early: younger than 11 years old
• Very early: younger than 9 years old
• Late: aged 16 and above.

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

The trends were strongest among participants belonging to racial minorities

The average age at menarche was found to decrease as age decreased. Participants born between 1950–1969 had an average age at menarche of 12.5 years old, compared to participants born in 2000–2005, who had an average age of 11.9 years old. The rates of early and very early menarche also almost doubled in these age categories from 8.6% and 0.6%, for the 1950–1969 group, to 15.5% and 1.4%, for the 2000–2005 age group, respectively. In these groups, the percentage of participants who had regular menstrual cycles within 2 years of their first period decreased from 76% to 56%.

These trends were present among all socioeconomic groups; however, they were more common in participants who identified as Black, Hispanic, Asian or mixed race and those belonging to a low socioeconomic status.

“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.

BMI may impact age of menarche

Childhood obesity is a growing epidemic in the US and many other countries. An increased BMI is considered a risk factor for early puberty and may contribute to this change in trend of age at menarche. The researchers suggest that 46% of the temporal trends in menarche can be explained by an increase in BMI.

However, BMI does not explain the remaining 54%. Previous research has also shown the biggest decrease in age at menarche happened before the obesity crisis in the US. Further studies are needed to explore various other factors that may be contributing to this change in trend, including environmental factors such as exposure to endocrine-disrupting chemicals, dietary patterns and psychological stress.

Dr. Michelle Wise, an associate professor in the Department of Obstetrics and Gynaecology at the University of Auckland commented on the impacts of the study: "In my practice as a gynaecologist in a large public hospital, we see many women every day with irregular menstrual cycles, and our routine questioning includes age of menarche and how long it took for cycles to become regular.”

“In the gynaecologist office, we see heavy menstrual bleeding resulting in anaemia 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,” said Wise.

“Continuing to investigate early menarche and its drivers is critical,” said lead author Dr. Zifan Wang, postdoctoral research fellow in 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.”

Reference: Wang Z, Asokan G, Onnela JP, et al. Menarche and time to cycle regularity among individuals born between 1950 and 2005 in the US. JAMA Netw Open. 2024;7(5):e2412854. doi: 10.1001/jamanetworkopen.2024.12854

This article is a rework of a press release issued by Harvard University. Material has been edited for length and content.