You may have heard of or even think you’ve experienced the phenomenon of period syncing, which is when the menstrual cycles of two women who spend a lot of time together synchronize. Find out what exactly period syncing is, whether it’s a myth, and why so many women claim they have experienced it.

Period syncing, also referred to scientifically as menstrual synchrony, is the much-debated idea that women who spend a significant amount of time together eventually start to experience menstruation onset at the same time or closer to the same time than they did before living or being in close proximity.

The idea was originally introduced in 1971, in a study that found women living together in a college dorm had more synchronized periods compared to random pairings of women (McClintock 1971). 

The explanation for period syncing put forward in this original study is that pheromones of women in close proximity interact and influence one another. Evolutionary biologists built on this idea to posit that perhaps period syncing helps reduce the chances of multiple females being impregnated by a dominant male since women with the same menstrual onset would be fertile at the same time. 

This idea that women’s bodies could cooperate to outsmart an attempt at male domination fed into the wave of feminism at the time and gained widespread popularity. Meanwhile, the evidence remained lacking – in studies investigating this possibility in baboons, our close relatives, researchers concluded that menstrual syncing is more likely to be explained by chance than by an evolutionary strategy (Clarke et al., 2012). 

Researchers have proposed alternate reasons for why syncing may occur, including that phases of the moon have an impact on the onset of menstruation. Although the idea of menses and the lunar cycle has been intertwined for centuries, the evidence does not support this theory (Ilias et al., 2013).

So, Is Period Syncing a Myth?

If you’re wondering, ‘is period syncing a myth?’, then you’re not alone. You’ve likely had a conversation with a friend at some point during which one person mentions they’ve just gotten their period and you realize that you’ve started yours too. It’s natural to attribute this to period syncing with your close friend.

According to the current evidence, chances are that instances like these are purely coincidental. Most studies that tried to replicate the original experiment failed to find proof of period syncing (Ziomkiewicz 2004). Many studies that explored this phenomenon (including the original study) were eventually discredited due to methodological flaws (Wilson, 1992). 

So, if there’s no evidence that period syncing is a real thing, then how do you explain having a period at the same time as someone you spend a lot of time with? In fact, it’s not so surprising that period syncing would happen. 

An average menstrual cycle is 28 days, which means that the maximum that two menstruators can be out of phase is 14 days. Since most women menstruate for 5 days per cycle, it is probable that you would overlap with friends who are also menstruating to some degree. If your menstrual cycle changes and that just happens to sync up with a friend’s, remember that other possible factors can influence your menstrual cycle.

Influence of Environment and Lifestyle on Periods

Several studies have shown that the length of a menstrual cycle is sensitive to environmental and lifestyle factors. Environmental factors include exposure to pollution and endocrine-disrupting chemicals. Lifestyle factors include exercise, sleep patterns, diet, alcohol intake, drug use, and stress (Campbell et al., 2021). 

Impact of Hormonal Birth Control on Periods

When you begin using hormonal birth control or change the type of birth control you are using, it is likely to cause some changes to your menstrual cycle (Hubacher et al., 2009). This is one way to explain a change in your cycle when it happens to look like period syncing is occurring.

Now that you’re all caught up on period syncing, the next time someone thinks they’re period syncing with a close friend, you can let them know that the scientific research simply doesn’t support this. 

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Editor: Lanna Last & Thomas Sauls