I’ve always been told the idea that women “sync up” with other women’s menstrual cycles is a myth. But I have sisters, and I swear when we’re together for a few months we sync almost to the day. Many of my female friends have corroborating stories of their own. Can it really just be coincidence?

— Anonymous, Brooklyn, N.Y.

[Have a question about women’s health? Ask Dr. Gunter yourself.]

Short Take

There is no science to support the claim that menstrual cycles of women who live together or who otherwise spend a lot of time with each other sync up.

Tell Me More

The theory that menstrual cycles sync up for women who are nearby for extended periods has been around for quite some time, but the few studies that support the theory are of low quality. In addition, no one has been able to suggest a plausible mechanism for such syncing up to occur.

In 2017, the menstrual period tracking app Clue evaluated data from its users to see if there was any link between menstrual cycle schedule and time spent together and found no association.