This Is the Real Reason Women Get Their Periods at the Same Time

Almost every woman I know has told me it's happened to them. You start spending a lot of time with another female, and your periods seem to magically sync up. In a way, it's oddly comforting: our bodies seem to "know" that we're better off experiencing the painful, stigmatized, and startlingly expensive mess that is menstruation together. According to one study, 87 percent of women have heard of this "menstrual synchrony," and a full 70 percent say they've personally experienced - and even enjoyed - the show of solidarity.

If you lived in 1970s, there was seemingly unequivocal evidence that this phenomenon was real. That's because in 1971, an undergraduate named Martha McClintock published a then-landmark study that found that in an all-female dorm, pairs and groups of women who spent the most time together seemed to synchronize their menstrual cycles. Believers in the "McClintock effect" postulated that synchronization occurs because of lunar cycles; others suggested it was due to pheromones, chemical factors secreted by animals that trigger behavior change in nearby companions.

But when other scientists tried to re-create McClintock's findings, things got messy. In one follow-up study, researchers examined the menstrual cycles of lesbian couples and found no evidence that their cycles synchronized.

In a string of subsequent investigations, the bulk of the evidence appeared to contradict McClintock's initial findings; only a minority of studies - most of which had methodological flaws - found support for the synchrony hypothesis. Then, this year, in the largest study to date, scientists at the University of Oxford used the fertility app Clue to track the menstrual cycles of 360 pairs of women. Analysis of the vast majority of those pairs showed that their menstrual periods actually diverged over time.

It turns out that menstrual synchrony, in the way that McClintock first described it, isn't a thing.

But that doesn't mean that women don't frequently get their periods at the same time. In reality, coincidental synchronicity probably occurs with some frequency. But that's because, mathematically speaking, there's a high likelihood that your menstrual period will converge with your friend's a good deal of the time, thanks to differences in cycle length - often up to 10 days - between women and the fact that many women menstruate for up to five to seven days, or roughly 20 percent of each month. It's not surprising, then, that two women who are close to one another will often discover that they're both burning through tampons at the same time. Their cycles are continually diverging and converging, creating the appearance of synchronicity at times of convergence.

So if you've noticed that you're repeatedly on your period at the same time as your sister, your roommate, or your friend, you're not crazy. But the explanation probably doesn't lie in lunar cycles or pheromones. It's just math.

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