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Do Menstrual Cycles Actually Sync?

Just like your activity tracker syncs up with your mobile device, some women feel their menstrual cycles do the same with other females. Ask a woman that question and I am sure many will have a story about a time their menstrual cycle “synced up” with a sibling, best friend, coworker, or college roommate. You might think that with this many stories and anecdotal evidence that this actually happens. But let’s take a look at what the science says:

Menstrual Synchrony

In general, menstrual synchrony is when womens’ cycles “sync up” or align closer together over a 4 to 6-month interval. This may cause women to get their periods at or around the same time. However, this synchrony is just a widely-known wives’ tale with no strong scientific evidence to back it up. Proximity and living together does not lead to changes in cycle timing or frequency. Periods just don’t work that way.

In 1971 a graduate student named Martha McClintock published an article in the science journal Nature titled, “Menstrual Synchrony and Suppression.” The target population for her study was women in her dormitory, and her findings showed that over time, women who lived together tended to cycle together. Since then, additional studies were performed using a variety of female groupings—c ollege roommates, same-sex female couples, and roommates who are close friends. But these researchers were not able to duplicate findings of menstrual synchrony.

Reasons Women’s Menstrual Cycles Might Synchronize

By Chance

Women’s cycles have variable lengths; a “normal” cycle length is considered to be anywhere between 21 and 35 days and most menstrual flows last between five and seven days. Therefore, it would not be uncommon for women to have overlap on their bleeding days.

For example, if two women have cycles that are 28 days long, the maximum they could be out of sync would be 14 days. On average, they will be seven days apart, and half of the time their period start dates should be even closer. And since a woman’s period typically lasts at least five days, it is not surprising that friends commonly experience overlapping menses, which is taken as personal confirmation of menstrual synchrony.

Changes in cycle length due to lifestyle/stress

Women with missed or erratic ovulations have the most variability and may report their periods being a few days early or late. Over time this may cause their period to align with another female they’re close with, as one woman’s cycles constantly vary from month to month.


A pheromone is a chemical substance that gives a subtle olfactory (smell) message to another member of the same species. However, studies do not show consistent findings that pheromones affect menstrual synchrony. 

Myth vs. Reality

Women “syncing” or cycling together over time is an idea that many would like to believe. But the studies that have been performed so far (even by the same researchers) do not provide the consistent, repeatable results that prove this is true. And this may be a good thing—isn’t it better if we’re not all having PMS symptoms and menstrual cramping all at the same time?