Why Do My Periods Sync Up with Others’?

The idea that women who live together have periods together is older than history books, but science has shown that there is no invisible force that unites women in this way.

In the early 1970s, scientists believed that pheromones—odorless vapors that send messages between animals—were the reason women’s menstrual cycles aligned over time.

However, research has now made it clear that, contrary to popular belief, period syncing simply does not happen any more often than by chance alone. The average menstrual cycle lasts 29 days, although there is huge variation between women, and cycles can range from 15 to 45 days. Intervals between periods also vary month to month, so two or more women will invariably have some months when they are in sync and others where they’re not. It’s the times when your periods fall on the same days that tend to stick in your memory—it’s as simple and unmystical as that.

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just coincidence

The calendar shows how the monthly cycles of four women vary from month to month and naturally fall in and out of sync over a four-month period.