Our scientific understanding of the world is always changing and adapting to accommodate new information and discoveries. However, some misunderstandings or misconceptions come up so regularly that they become taken for granted as fact, despite evidence to the contrary. Did anyone else learn that humans only use 10% of their brains? Or that most DNA is “junk DNA” that doesn’t serve any function? Or that eight glasses of water a day is the right amount for everybody? One misconception you can find across textbooks, puberty brochures, and even well-respected medical organizations (like the Cleveland Clinic!) is that the average menstrual cycle is 28 days long, and that ovulation generally occurs on day 14 in a 28 day cycle.
But a fascinating study by the fertility awareness app company, Natural Cycles, recently analyzed over 600,000 menstrual cycles to show us what the real averages are, plus how age and BMI affect our cycles [1].
Where did this menstrual cycle data come from?
The FDA-approved fertility awareness app, Natural Cycles, has over 124,000 users. The app primarily relies on body temperature readings (gathered via the Natural Cycles thermometer, Oura Ring, or Apple Watch) to gauge when ovulation likely occurred. App users also have the option of tracking additional fertility signs like cervical mucus or luteinizing hormone (LH) levels to make using the app more akin to the symptothermal method.
For the Natural Cycles study, cycles were included in the analysis if they met the following criteria: 1) The user had logged at least six cycles; 2) the cycle did not include a pregnancy (as a year-long “cycle” would really skew the data); 3) the cycle logged was not more than 90 days or less than 10 days; and 4) users logged temperature on at least 50% of the days, so that a day of ovulation could be estimated. In total, 612,613 cycles met these four criteria. While personal identifying information was removed from the final analysis, data such as user age and BMI were included to identify associated trends.
What trends were observed?
Before I dive into the results of the study, let’s do a quick review of the menstrual cycle:
- The first day of a period is considered Day 1 of a new cycle.
- The follicular phase is the time between the first day of the cycle to ovulation.
- The luteal phase is the time between ovulation to the last day of the cycle (the day before the next period starts).
- The timing of ovulation can vary from cycle to cycle, and can be influenced by stress, nutrition, sleep, and how the other hormones in your body are functioning.
The average cycle length wasn’t 28 days
In the Natural Cycles study, the average cycle length was 29.3 days–slightly longer than the generally accepted 28-day average. The average follicular phase length was 16.9 days, with a range from 10 to 30 days. The average luteal phase length was 12.4 days, but ranged from 7 to 17 days. This means that, on average, ovulation did not happen in the middle of the cycle, but slightly past it (about 57.7% of the way through it, to be a bit more exact!).
This means that, on average, ovulation did not happen in the middle of the cycle, but slightly past it (about 57.7% of the way through it, to be a bit more exact!).
As the researchers noted in their analysis, this finding is significant for women attempting to identify their fertile window by cycle length alone. Calendar-based cycle tracking apps (and medical advice) that assume a 14-day follicular phase could lead women to partially or entirely miscalculate their fertile window, with obvious implications for anyone seeking to avoid pregnancy. This highlights the importance of learning to observe and track individual biomarkers (like basal body temperature and cervical mucus) via an evidence-based fertility awareness method, especially for those trying to achieve or avoid pregnancy.
As women age, their cycles tend to shorten
The Natural Cycles study also noted age-related differences in cycle variability (i.e., how much an individual’s cycle length changes from month-to-month), cycle length, and follicular phase length. For the ages included in this study (18-45), variation was highest in young adulthood (that is, age 18 until about age 23). Afterwards, variation decreased until around age 40, when it gradually began to increase again. This follows what we would expect to see as the body’s natural rhythm is still being established in the early years of cycling, followed by a period of relative stability in cycle length, which is followed again by a period of variability as older app users begin to experience perimenopause.
Researchers also noticed a very steady decrease in cycle length between the ages of 25 and 45, with average cycle length decreasing by 0.18 days with each year of age. While a correlation between age and cycle length has been noted in other studies, this measurable, linear decrease is a new observation. The cycle length average for 25-29 year olds was 29.9 +/- 5.5 days, and 27.4 +/- 5.2 days for 40-45 years olds. This means that, based on this data, you could expect your cycles to become 2 to 3 days shorter over the course of your fertile years. Luteal phase length did not seem to vary with age, however, so the gradual decrease in overall cycle length was due to the gradual decrease in follicular phase length.
Do Natural Cycles app users represent the average woman?
While this study boasted a large sample size, there are some factors that may prevent the study from being generalizable to the average woman. For one, Natural Cycles is an app that requires the use of a smartphone and either a bluetooth thermometer, Apple Watch, or Oura ring. It also requires the user to pay either a $15/month or a $120/year subscription fee. This naturally creates a barrier to use for women with limited income or for those who simply prefer to use FAM for free. These financial factors could impact the generalizability of the data.
Natural Cycles also uses either basal body temperature (BBT) measurements with their branded oral thermometer or body temperature measurements via the Apple Watch or Oura ring as its main fertility biomarker. It does not require cervical mucus observations, which might deter experienced FAM users from using the app, prompting them to use an app that focuses more on cervical mucus, instead. Basal body temperature readings are also subject to human error and can be impacted by sleep and lifestyle factors.
For example, I used to track BBT while at a job that led me to have a very irregular sleep schedule, and my temperature charts could sometimes be erratic as a result. In fact, the Natural Cycles researchers noted that about half of the cycles in their database (out of 1.4 million total cycles) could not be used in the analysis because the app was not able to estimate a day of ovulation for those cycles (usually due to insufficient user data entry).
What does this study teach us?
In some ways, this study highlights what fertility awareness proponents have known all along, and what the 2022 Apple Women’s Health Study had already confirmed: the “standard” 28-day cycle with a 14-day follicular phase doesn’t apply to most women. This is why “calendar-only” or “rhythm method” tracking systems that rely on this standard without a double-check (like BBT or cervical mucus or urinary hormone levels) are unreliable for preventing pregnancy. This study found that ovulation tended to happen later (around day 17) than traditionally assumed, and that for 18-24 year olds, the average follicular phase length was actually 18 days. For women who want to understand their fertile window to avoid or achieve a pregnancy, having an accurate understanding of when ovulation occurs is crucial.
As I’ve touched on in this article, Natural Cycles wouldn’t be my first choice for understanding my cycles, both because of cost and its sole reliance on BBT. However, I do applaud them for promoting a hormone-free approach to understanding fertility, and for making this data available for research. My hope is that this study will spur many more large-scale analyses of naturally-cycling women–and hopefully ones that will measure more fertile signs and population groups.
And, if you aren’t already using FAM, my hope for you is that the rise of Femtech apps and devices prompts you to become curious about your own cycle. Find a method that works best for you, your goals, and your lifestyle, and start reaping the many benefits of fertility awareness!
Additional Reading:
FAM Basics: Why you really need to learn from a fertility awareness method instructor
Interview with Dr. Elina Berglund, founder of the FDA-cleared Natural Cycles app
