Flo research shows how menstrual cycles and symptoms change with age in the largest study of its kind

Medically reviewed by: Adam Cunningham, PhD, Flo senior research scientist, UK

Menstrual cycles naturally evolve across the reproductive lifespan, yet clinicians have historically had limited large-scale data describing how cycle length, regularity, and symptoms change with age, particularly beyond 45, when perimenopausal changes typically emerge.

To address this gap, Flo’s research team analyzed cycle and symptom data logged by millions of app members to characterize age-related menstrual patterns in unprecedented detail. Our study examines how cycle length, variability, period duration, and common symptoms change from early adulthood to the perimenopausal years.

This study has been published in the scientific journal Scientific Reports, part of the prestigious Nature Portfolio publishing group. 

Study overview

More than 19 million members of the Flo community, aged between 18 and 55, contributed data to this analysis. This makes it the largest study of menstrual cycle patterns and symptoms to date. Until now, most studies have only included women up to the age of 45, leaving limited evidence to describe menstrual changes beyond this age. 

Adam Cunningham, PhD, senior research scientist on Flo’s science team and lead study author, said, “It was surprising how little research there is about menstrual cycles and symptoms, and most research stops at 40 or 45.”

“Given that we have the largest dataset about menstrual cycle patterns and symptoms at Flo, we wanted to rise to this challenge and provide much-needed data about women’s experiences, particularly in the over-45 age group,” Cunningham added.  

In collaboration with Dr. Lubna Pal, professor of obstetrics and gynecology at Yale School of Medicine and a member of Flo’s Scientific Advisory Board, the Flo science team analyzed cycle and symptom data from over 19 million app members, including 448,910 aged 46 to 55. The analysis examined age-related trends in cycle length, menstrual cycle duration, cycle irregularity, and frequently reported symptoms.

Key findings

Clear age-related differences in cycle length were observed. Mean cycle length increased from adolescence into the early 20s, peaking at approximately 29 days around ages 21 to 22. Cycle length then gradually decreased through the mid-reproductive years, before increasing again during the perimenopausal years, a pattern clinicians often observe in practice.

Between the ages of 22 and 45, the cycles gradually shorten by approximately two days. After age 45, cycle length increases again, consistent with the hormonal changes associated with the perimenopausal transition. 
Cycle variability also showed clear age-related patterns. Among those aged 18 to 25, cycle length typically varied by around 4.1 days from cycle to cycle. Variability decreased and reached its lowest point between ages 36 to 40, before increasing again. In those aged 51 to 55, cycle length fluctuated by approximately 6.5 days on average.

Nearly half of the study participants aged 51 to 55 and nearly 1 in 3 aged 46 to 50 met the criteria for cycle irregularity as described by the American College of Obstetricians and Gynecologists. This pattern is consistent with the expected increase in variability during the perimenopausal transition.

Irregular cycles were the least common in those aged 36 to 40, where just over 1 in 10 experienced them.  

While menstrual changes are often attributed to perimenopause, our study found that period durations are longest between ages 18 and 25, with an average duration of 5.2 days. Periods then gradually get shorter and last, on average, only 5.06 days in those aged 41 to 45. After that, periods become longer again, but — importantly — they also become much more variable in length than in any other age group. 

Age-related symptom patterns

Over 12 million of our members logged symptoms in the Flo app, and we found that these varied by age. 

Cramps were the most commonly logged symptom across all ages. Tender breasts and fatigue were also among the top three symptoms up to age 45. After 45, headache became more common and replaced fatigue in the top three.

Older users were less likely to log that they’d experienced cramps or acne around the time of their period. Headaches and backaches were more common among these users. In contrast, logging of tender breasts and fatigue was highest in users’ late 20s and early 30s.

The Flo community didn’t just log physical symptoms. With increasing age, reports of stress and insomnia were more common, as was logging of a happy mood, which peaked at age 50. Mood swings were most frequently reported by the youngest and oldest age groups. 

Bloating was by far the most commonly logged digestive symptom in all age groups. But constipation was noticeably more common among older members. 

Implications for clinical practice

Dr. Lubna Pal said: “The importance of this study lies in the chronicling of menstrual cycle and symptom data for a large and diverse population of women spanning in age from early adulthood into perimenopausal years. Findings from this epidemiological approach based on real-life data allow a more comprehensive perspective on the prevalence of menstruation-related phenomena and if and how these may vary across stages of reproductive aging. The wide-ranging populations’ perspectives offered in this study can help enhance population awareness of what symptoms, if being experienced by a majority, could even be construed as “norm,” may even help lessen the trepidations and stigma attached to aging, and can enhance preparedness of health care givers in their approach to inquiry, assessment, and efforts at optimizing care of women across the lifespan.”

This research was made possible through the anonymized symptom and cycle data recorded by Flo app members. Their engagement has provided a uniquely detailed view of menstrual patterns across adulthood and the perimenopausal transition.

We acknowledge and appreciate the Flo community for their continued contribution to real-world evidence generation.
With one of the largest datasets ever analyzed in menstrual health research, these findings help contextualize common patient presentations, from changes in cycle length to symptom patterns around perimenopause and can support clinicians in framing what may be typical during reproductive aging.

The published study can be found here: https://www.nature.com/articles/s41598-024-60373-3.

Discover how Flo supports clinicians with evidence-based patient education and a single, consolidated health report that brings cycle, symptom, and mood data together in one place. Visit About Flo for Healthcare Professionals to learn more.