Science Has Just Debunked the Myth You’ve Always Believed About Periods

Do you and your BFF always get your periods at the same time? It might just be nothing more than a coincidence, says science.

Maybe you and your mom, sister, or best friend seem to always get your periods together, sharing cramps and tamps. But one study says those synched cycles are all in your head.

University of Oxford researchers paired up with period tracking app Clue to figure out if women’s menstrual cycles actually sync with the friends, partners, and family members they spend the most time with. It was the biggest study of its kind, and turns out, it’s all a myth. (But since period pain is very real, these treatments are worth the try.)

The researchers found 360 pairs of women who knew each other, lived near each other, and didn’t use hormonal birth control. After tracking each pair through three cycles, the researchers found the gap between periods of 273 pairs actually started drifting further apart. Only 79 of the pairs synched closer together.

Do-Women’s-Periods-Really-Sync-Up-Roman Prishenko/Shutterstock

Living together didn’t increase the odds of synching up either. Less than one quarter of the pairs who did start synching lived in the same home. On the other hand, 37 percent of the pairs whose cycles got further apart lived together.

Where did the myth come from anyway? The idea that women’s cycles match up might have gained traction from a 1971 study claiming college dorm residents started synching with their best friends and roommates. Since then, though, that study has come under fire for faulty methodology, and other studies have found conflicting results.

“But wait!” you might be saying, “My best friend and I always seem to be bleeding the same time!” Sorry, but even though it might be a bond for you two, you’re probably programmed to notice only when things work out that way.

“When I say to my friends that I have my period and they have theirs too, I don’t conclude that we are synching,” Marija Vlajic, Clue’s data scientist, tells The Guardian. “I think it’s just information bias, our brains looking for patterns.”

But hey, if your period is an excuse for a night in with the girls—complete with chocolate and sweatpants—we’re all for it. “I can see how it gives you a special connection with a woman to go through that at the same time,” Vlajic says. “It feeds into a feeling of connection, support, and sisterhood.”

MORE: 8 Period Problems You Should Never Ignore

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Marissa Laliberte
Marissa Laliberte-Simonian is a London-based associate editor with the global promotions team at WebMD’s and was previously a staff writer for Reader's Digest. Her work has also appeared in Business Insider, Parents magazine, CreakyJoints, and the Baltimore Sun. You can find her on Instagram @marissasimonian.