Why Do Women's Periods Sync With Their Girlfriends'? The Truth About Whether (Or Not) We Can Time Our Menstruation Cycles

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When I was growing up, my mother and two older sisters would get their periods at the same time, and my dad would call it "the Devil's trifecta." Of course, I thought when I got my period I would just be penciled into the schedule, but it just doesn't happen like that.

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So what's the truth, if any, behind the idea that if a group of women spends enough time together their periods will sync?

It first came about in 1971 when American researcher Martha McClintock decided to study the menstrual cycles of 135 women at Wellesley College. McClintock found that friends and roommates had more similar cycles than random pairings of women. She came to the conclusion that it was all due to signals that are given off by pheromones. Think of it as one woman who's a beacon of pheromones, and everyone else's cycles are attracted to hers.  

While tons of women can vouch for this experience, a lot of scientists haven't been able to replicate the experiment, meaning that it could have just been a fluke. Luckily, thanks to period tracker apps like Clue and Eve, it's easier to run new experiments. 

After analyzing data from these apps, it was found that 79 percent of the pairs of women who claimed they were synced with a friend actually were. But, it was also found that their cycles tended to diverge over time rather than converge.

More importantly, they found that living together didn't increase the chances of being on the same cycle at all. 

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Researcher Jeffrey Schank said that coinciding cycles could all just be a coincidence. Since cycles vary in length from person to person, it's hard for them to occur at exactly the same time. It's a lot more likely that the overlap between my mom and two sisters' cycles was just random rather than a definite thing. Especially since we all experience little shifts in our periods — such as starting a day late or ending a day early here or there — it's extremely hard to keep on the same cycle with someone else for prolonged amounts of time. 

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So, if your period is late or shorter than usual it could be a lot of things like a new workout routine or prescription, but it definitely isn't due to your new group of friends or workplace. 

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Josie Fuller is a writer who studies Journalism and Women's Studies at The University of Florida. She writes about pop culture, trending topics, celebrities and astrology.