Women report it anecdotally, but the science might not be there yet.
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When I was in college living with two other women, they swore to me that their periods had shifted to align with my cycle. Up until spending most of their time with me, they'd had their periods at one point in the month and now they were getting theirs when I got mine instead. We decided that our periods had synced up.

Many women have stories like mine, whether it's with college roommates, close friends or family members. But is syncing up really a thing? Is there actual clinical evidence to suggest that women who spend a lot of time near each other would start to get their periods at the same time? The short answer is maybe -- researchers call it "menstrual synchrony." As of now, there's no foolproof evidence that it's a real physiological phenomenon, but there have been a few interesting clues.

What exactly is "menstrual synchrony"?

Menstrual synchrony is a theory that women who live near each other or spend a lot of time together will start to see their cycles sync up so that they get their periods around the same time. Pheromones, or "airborne chemical signals that are released by an individual into the environment and which affect the physiology or behavior of other members of the same species," have been thought to play a role.

The concept was first documented in Martha McClintock's seminal research paper in 1971. McClintock observed 135 female Wellesley College students living in the same dorm and found "a significant increase in synchronization of onset dates" among the women who spent a lot of time together. According to these findings, it's not just an anecdotal phenomenon -- women do sync up.

So, can women's periods really sync up?

While some of the researchers who followed McClintock were able to replicate her findings, many didn't find evidence of menstrual synchrony. In 1992, researcher H. Clyde Wilson found errors in McClintock's study model, specifically her sample selection and method of determining period start dates at the beginning of her study. When he corrected for those errors, he found no significant evidence of menstrual synchrony.

The issue is that women's periods when they're not on birth control might be too inconsistent to truly sync up. The prevailing theory now is that women's synchronicity is, as researchers Zhengwei Yang and Jeffrey C. Schank put it, "at the level of chance."

In their 2006 study on the topic, Yang and Schank wrote that "cycle variability leads to repeated cycle convergences and subsequent divergences, which may explain the perception of synchrony." Meaning: Women might think they've synced up their periods when they may have only randomly aligned for a short amount of time and will likely phase in and out of their matched-up schedules.

But what if you really, really think you've experienced it?

While there's no solid evidence of menstrual synchrony at the moment, that doesn't mean the book is closed on syncing up. Researchers are still looking into the the theory, and women keep reporting their own non-clinical experiences with it. So hey, if you think it's a thing for you and your friends and family members, then why let researchers spoil the fun? A little period solidarity never hurt anyone.

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