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You’ve probably heard that women who live or work together tend to wind up on the same cycle. Friends have told me how their coworkers all seem to complain of menstrual cramps at the same time, and the tampon box in the women’s restroom empties regularly each month like clockwork. And books such as The Red Tent popularize the belief that our menstrual cycles sync with the women close to us.

While it’s often stated as a fact, you might be wondering: Do women’s periods actually sync up? Or is it merely the stuff of urban legend?

Past Findings

Studies on women’s menstrual cycles and how they did or didn’t sync have been conducted for decades. One of the earliest examples is a 1971 study widely credited with “proving” that women who live together or are close friends tend to be on a similar cycle and attributed this to chemical signals given off by the body known as pheromones. However, the study’s statistical design was criticized because while a few researchers were able to replicate the results in future studies, other researchers were unable to replicate them. Even though the study itself had holes, the myth has persisted.

The Research Today

More recently, researchers used data from the period tracking app Clue to investigate whether there is evidence for this phenomena. The study looked at 360 pairs of women who used the app and who reported that their cycle was syncing with someone they knew who also used the app. After analyzing the data, the researchers found that 273 of the 360 pairs actually experienced their cycles differing rather than syncing. Only seventy-nine of the pairs saw their cycle start dates get closer to one another. The researchers found that the women’s cycles were more likely to diverge over time rather than converge. They also found that living together did not increase the likelihood of women’s periods syncing.

Blame the Coincidence on Our Cycle Lengths

In an interview with Scientific American, researcher Jeffrey Schank observed that because women’s cycles are often different lengths, it’s difficult for them to occur at the exact same time. He theorized that any similarity in cycles between women is likely more random and less likely to remain the same over time because of the small shifts in their cycle lengths. And, in his own study—the longest done on menstrual synchrony to date—he found no evidence of periods syncing up. Instead, Schank and his team found only random overlaps. Given that there are so many factors involved in the length of a woman’s cycle and her fertility (e.g., miscarriages, pregnancy, postnatal, stress, exercise activity, weight loss or gain, and nutrition), it’s easy to see that there is a great deal of variability in an individual woman’s cycle that would make it difficult for her menstrual cycle to synchronize consistently with the women she is in close contact with.

So as it turns out, more and more research findings point to evidence that women’s periods don’t actually sync, and anytime your cycle coincides with a friend’s, it’s most likely due to chance. Our bodies are as complex as our menstrual cycles. While it would be a cool (and perhaps useful) phenomenon, it would require the stars perfectly aligning for your period to consistently sync with your female coworkers, relatives, and closest girlfriends. But don’t worry, there are plenty of other things you can bond over with the women in your life!