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Menstrual Cycle Length and Patterns: The University of Adelaide and Flo Study Results

Recent research was conducted as a collaborative effort between Flo female health and wellbeing app and world-renowned researchers from the University of Adelaide’s Robinson Research Institute (RRI). The following article summarizes the results of this new study on factors that influence menstrual cycle patterns and female reproductive health.

An article containing the results of this research, which describe the demographic, lifestyle, and menstrual cycle characteristics among a large global group of women, assessing differences across age and body mass index (BMI), was published in the Journal of Medical Internet Research (June 2020). 

One of the most surprising findings for us at Flo was that although female health and menstruation have been and continue to be well-studied, there is far less information available about how lifestyle differences can affect the menstrual cycle. 

This study included information from over 1.5 million Flo app users ages 18 and over who had logged at least three menstrual cycles in the app. 

Data related to demographics, menstrual flow, cycle length, ovulation information, and reproductive health and diseases were collected from self-reporting only. 

The privacy policies at Flo are strict, and researchers were only permitted to use de-identified, aggregated data from the app for this study. 

Read on, and you will see the results that were uncovered in the researchers’ findings. 

So, how long is the average menstrual cycle? 

The average cycle length has previously been understood to be 28 days. However, it has been shown through this study that a 28-day cycle is fairly uncommon — only found in 16 percent of all participants. It’s thought that a variety of differences among people (age and BMI) and lifestyle factors (stress, physical activity, etc.) can cause variations in the average cycle length, as well as the average period length.

The findings from this study show a direct correlation between age and median cycle length. With increasing age, the median cycle length tends to be shorter.

For example, it was more common for women 40 years and over to have an average cycle length of 27 days than for younger participants aged 18 to 24, who tended to have an average cycle length of 30 days.

Although age was related to the cycle length, it was determined that increased BMI did not make a significant difference in average cycle length among users.

Women 35 years and over tended to have a less variable cycle (mostly within 1.5 to 4.5 days) than younger participants, among whom cycles with greater variability were quite common.

It was also found that participants with a higher BMI had less variation in their cycle.

It was shown that younger women (18–24 years old) experienced a higher number of menstrual cycles with short luteal phases (5 to 10 days) and fewer number of cycles with long luteal (15 days and more) phases than older women (40 and over), for which the opposite situation was typical.

Only 13 percent of cycles within this study had an estimated ovulation on day 14. This points to much greater variation in the follicular and luteal phase lengths than previously thought.

Increased BMI did not make a significant difference in the length of the follicular phase and the length of the luteal phase among users. The data did show that only those with a BMI greater than or equal to 50 kg/m2 in the study experienced changes in these measured outcomes.

What does the study say about how lifestyle influences period and cycle? 

According to the current results, both stress and physical activities can play a role. 

For example, a higher percentage of women with short cycles (20 days or less) reported experiencing high levels of stress on a regular basis and not exercising compared to women with normal and long cycles.

Surprisingly enough, habits such as alcohol consumption and smoking didn’t show any significant relation to menstrual cycle length.

This collaborative study between Flo and the University of Adelaide’s RRI showed interesting results:

  • The median cycle length of 28 days was only found among 16 percent of this study’s participants. 
  • Younger study participants experienced a greater number of menstrual cycles with short luteal phases when compared to older participants, who had a higher number of longer luteal phases during their cycles.
  • Only 13 percent of study subject cycles had an estimated ovulation on day 14. 
  • BMI wasn’t found as an important factor which impacts cycle length.
  • Stress and sedentary lifestyle are found to be related to shorter cycles.

These new findings are vital to support obstetric clinical guidelines and recommendations for patient care related to menstrual cycle length and patterns. This is particularly important when considering fertility treatment options. 

Grieger, Jessica A, and Robert J Norman. “Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study.” Journal of Medical Internet Research, JMIR Publications Inc., Toronto, Canada, 24 June 2020, www.jmir.org/2020/6/e17109/.

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