You can probably point to that one girlfriend who always seems to have her period at the same time as you. The idea that periods sync has been around forever. I always saw it as one of those cool, mysterious things about women: We’re so in tune with our best friends that our menstrual cycles actually align! But it turns out this phenomenon may be no more than a myth.
For a new study, the period tracking app Clue partnered with data scientists from the University of Oxford to follow the cycles of 360 pairs of volunteers who shared a close relationship-say, as sisters, roommates, or partners-for three months via the app. None of the participants were on hormonal birth control.
The researchers discovered that over time, the women's menstrual cycles were more likely to diverge than sync. After three months, almost 76% of the pairs had a larger difference in their cycle start dates than they did at the beginning of the study. And 100 of those 273 pairs actually lived together.
It's worth nothing that this study has not been published in a peer-reviewed scientific journal. But the findings do line up with prior research.
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There have been other studies on menstrual synchrony (as it’s technically called) that concluded period syncing isn't actually a thing. One from 2006, for example, tracked 186 Chinese women living together in dorms for a year, and found no evidence that their periods were lining up.
The concept of period syncing seems to have originated in a 1971 study published in Nature. The author suggested that one woman's pheromones might shift another woman's cycle. But that study has seen been criticized for its methodology.
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So why does it seem like you and your bestie get your periods at the same time? Alas, it’s most likely due to chance. Because our menstrual cycles vary in length, you and your friend are bound to overlap at times, and those times probably stick in your mind.