Does your period really sync up with women you live with?

Have you ever suspected that you and the women you see most seem to feel cranky, tired, or bloated around the same time each month? The idea of menstrual cycle syncing has been around for decades and some people have even tied the lunar cycle into the equation. But does data back up the claim that menstrual periods can really sync up?

According to a recent study carried out by the fertility tracking app Clue in partnership with the University of Oxford, data indicates that women’s periods do not synchronize after all. This study was the largest of its kind, tracking 360 pairs of women.

Researchers found that 273 of the pairs (a whopping 76%) reported having a greater time difference between their periods at the end of the study than they had at the beginning. Living together did not seem to increase the likelihood of cycle alignment either, according to the study.

Where did we get the idea of period syncing?

A small study by in 1971 suggested this idea. It tracked 135 women living in the same dorm room for a 4-6 month period. Some evidence was compelling, but later critiques in the early 90’s noted statistical errors in this study:

  • Failure to control adequately for the convergence of onsets by chance
  • An incorrect procedure for determining the initial onset absolute difference between subjects
  • Sampling biases

Still, many of us bought the claim and have been believing it ever since! Researchers suspect women kept this myth alive because it offers a feeling of connection, support, and sisterhood. Perhaps our fascination with period syncing may reveal more about the nature of women than the myth itself.

TL; DR: A recent study (and the largest of its kind ever conducted) indicates that period syncing is not founded in verifiable evidence—it’s merely a myth.