myth of period syncing

Is Period Syncing A Thing?

Ever lounged on the couch, eaten ice cream, and fought over the heating pad with your roommate?

There are pretty much only two reasons periods ever get discussed on prime-time television: first, to draw attention to their lateness, thus introducing a pregnancy storyline.

And second, to note that two characters’ menstrual cycles have synced, indicating that they’ve been bonding or spending a lot of time together.

Ever lounged on the couch, eaten ice cream, and fought over the heating pad with your roommate? Or perhaps you never need to worry about having ibuprofen or your favorite essential oil on hand, because you know your coworker will also have some around a certain time of the month.

The theory behind the syncing of menstrual cycles is that women's pheromones interact when they are in close proximity, causing them to have their period at the same time. Many women buy into it.

Still, we can’t help but wonder if it’s actually true. Is there scientific evidence or data that supports cycle synchronization when women spend a lot of time together? We have done some research. Read on to find what we‘ve learned.

The roots of the belief

As humans, we always like exciting stories. We want to explain what we observe by something that is meaningful. And the idea that what we observe is due to chance or randomness is just not as interesting.

Experts report that period syncing—or, more formally, “menstrual synchrony”—was introduced into the popular consciousness in 1971 by a researcher named Martha McClintock. Her study on the menstruation patterns of students at a women’s college, published in the journal Nature, tracked the period start dates of 135 women who lived together in a dormitory over a time frame of about six months.

The study claimed to find “a significant increase in synchronization (that is, a decrease in the difference between onset dates)” among roommates and among groups of women who independently identified one another as a “close friend.” At the beginning of the study, these friends averaged about six and a half days’ difference between period start dates. By the end, they averaged a little less than five.

Dr. McClintock hypothesized that this was because the women who were spending time together had the chance for their pheromones to affect each other.

The idea is that if women had synchronized cycles, they would all be fertile at the same time – so one man would not be able to reproduce with them all.

He can't manipulate all the females at the same time, so that's why it was believed to be a form of co-operation between females.

A bit of simple math

But, there were also studies that didn't find evidence of periods syncing. And people started to pick holes in the earlier research – critics identified problems in the people chosen for the study. The definition of when they were syncing was quite loose.

For decades, researchers have been poking holes in the study that introduced the concept of “menstrual synchrony.” Many people believe in it anyway.

According to International Gynecology and Obstetrics, menstrual cycles do not actually sync up between women who spend a lot of time together – new research has found it.

A new joint study, performed by period tracking app Clue and Oxford University, has debunked the myth of syncing periods.

Not only did the study find that periods will not sync over time between those who spend a lot of time together and those who live together, it actually revealed that they are more likely to become more separate.

Based on this study, your cycle is actually more likely to get out of sync than in sync with another woman’s period.

Scientists have also pointed out a fundamental flaw in the period-syncing logic.

Given a cycle length of 28 days (not the rule—but an example), the maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer.

Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses, which is taken as the personal confirmation of menstrual synchrony.

In other words, as experts explain, women have their periods at the same time as other women an awful lot, so it’s common to mistake menstrual overlap for menstrual synchrony. A quarter of the time, two or more women’s menses should be overlapping, based on random chance – that‘s quite obvious.

TV and movies certainly help maintain the popularity of the period-syncing myth. But to some extent, it survives because so many people want it to be true.

No matter how inaccurate the myth of period syncing may be, the idea that women’s bodies can fall into collective rhythms carries a certain mysterious, otherworldly appeal and, lending the myth more inertia, gives women a way to feel connection, empathy, and collective empowerment with other women.

Even though the research says women’s cycles don’t converge, that doesn’t mean you can’t enjoy the feeling of comfort, connection, or sisterhood that comes along with having your period at the same time as someone else.

So, keep knowingly nodding when a sister asks to borrow a menstrual pad. While your cycles may not be biologically syncing, you’re still going through the same thing.

Back to blog