The Myth of the Average Menstrual Cycle

wrote this on August 1, 2014

Did you ever learn that an “average” menstrual cycle is 28 days long? I know I did! As a teen, I was always super worried that all my cycles weren’t exactly 28 days long. I was totally convinced that there must be something wrong with me…surely each of my friends had a “normal” cycle, right?

Little did I know back then, I had absolutely nothing to worry about. The idea that a woman’s cycles should be 28 days long is a complete myth! If you add up the cycle lengths of all women and then divide that number by the total number of women, you’ll probably get something close to 28 days. But this population average simply cannot be applied on an individual level. That’s just not how our bodies work!

When teaching children about menstruation, it’s much easier to say that cycles are 28 days long and that ovulation happens on day 14. In the real world, however, there is lots of variation from woman to woman and even on an individual level. However, we’re not told that this variation is totally normal. Well, here you go: variation is totally normal!


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This “myth of the average cycle length” ends up causing a lot of confusion and often makes women and girls think that they’re abnormal for not having 28 day cycles. And many of us end up feeling like our cycles are “irregular” simply because they’re not all the same length. This can be particularly frustrating during puberty, because cycles during this time can be more varied in length than in adulthood. But again, this is totally normal. After a girl starts her period, it can take several years for her hormones to stabilize, which makes her first few years of periods more prone to small variations. Even adults aren’t immune to this. There are lots of factors that can affect a woman’s cycle length.

It can be frustrating thinking that your cycles aren’t “normal,” but there are ways to make cycle variations feel more predictable. Measuring your basal body temperature (or your waking temperature) each morning can let you know that you’ve ovulated, and your luteal phase – or the time between ovulation and your next period – will always be a very consistent length. This means that you can actually predict your next period with incredible accuracy once you have confirmed ovulation with your basal body temperature. Predicting your periods is actually pretty easy to do, and Groove’s iPhone app can help you out.

I have yet to meet a single woman who consistently has 28 day cycles, so don’t buy the myth of the average cycle!

Jennifer Aldoretta is the cofounder and CEO of Groove. She is an entrepreneur, engineer, and biohacker who is obsessed with periods, nutrition, hormones, and the microbiome.