Blood sisters: Do women's menstrual cycles really sync up?

Why you and your BFF’s periods might seem to line up.
(Credit: iStock/Getty Images)

There's something about menstruation – maybe the universal braving of cramps and bloating – that promotes a feeling of solidarity among women. Perhaps an extension of that is the feeling of closeness between friends who seem to have "synced" their periods – believing that the very fact you're menstruating simultaneously is because you've spent so much time together. But is there truly a possibility you might also be celebrating a biological connection?

It's called "menstrual synchrony" and according to a 1999 study, 84 per cent of women were aware of it and 70 per cent reported experiencing and enjoying it.

However, experts say it's likely not a real thing – and new research backs them up.

The origins of why people believe in menstrual synchrony can largely be traced back to a 1971 study on the subject by Harvard University psychologist Martha McClintock. Published in the journal Nature, McClintock's study, titled "Menstrual Synchrony and Suppression," looked at the menstrual cycles of 135 female undergraduate students between the ages of 17 and 22 years old who lived in the same dormitory. Tracking the women's cycles for one academic year, McClintock found that women who were either roommates or close friends experienced converging menstrual cycles.  

The study, which suggested pheromones might influence synchrony, was long held up as evidence that menstrual synchrony exists. The phenomenon was even dubbed "the McClintock effect." But in recent decades, the methodology behind McClintock's study came under criticism and later studies failed to find evidence of menstrual synchrony.

In 1992, now-deceased University of Missouri anthropology professor H. Clyde Wilson criticized McClintock's study for using incorrect procedure to determine period start dates and said her study, and others with similar findings, excluded certain subjects and data, in turn increasing the probability of finding menstrual synchrony.  

Still, McClintock defends her work.

"We have completed a lot of research since the initial observation, which has refined our understanding," said McClintock in an email to CBC Life. "We and others have shown that the key event is pheromonal control of the timing of ovulation in humans. One manifestation of this can be menstrual synchrony if conditions are right, and many other studies have confirmed this. But if conditions are not right, and it is not observed, this does not disprove the underlying pheromonal mechanisms."

While some women's health experts say there's still a need for more research before they rule out menstrual synchrony, the consensus amongst many experts is that menstrual synchrony is a myth.

"What's happened is as people began to implement better methods, the effects completely went away," said Jeffrey Schank, a professor of psychology at the University of California Davis.

In his 2006 study titled, "Women Do Not Synchronize Their Menstrual Cycles," Schank looked at the menstrual cycles of 186 Chinese women living in dorms for more than one year. He found that when menstrual synchrony did occur, it was by chance. Not all women have the same length of cycle – for example, some women might menstruate every 28 days, others 30 days, others infrequently, he said – so it makes sense that there will be some overlap.

"Experiencing closeness or convergence is something that's almost a mathematical fact that will happen, but it won't persist," he said. "When women say, 'I have synchronized,' that's absolutely, in a statistical sense, true."

Indeed, a new study by a period-tracking and fertility app, Clue, and researchers at the University of Oxford came to similar conclusions. The study analyzed the menstrual cycles of 360 pairs of women who said they were synced up with each other, over three consecutive cycles. The study found the majority of the paired-up women's cycles were actually diverging.

"It's very unlikely that cycle syncing is a real phenomenon," Clue's data scientist Marija Vlajic told The Guardian. "Menstrual syncing amongst the sample we had did not exist."

So while you might feel a sisterly bond with synced-up females over the agony and ecstasy of menstruation, just know it's probably thanks to serendipity – not science.


Katrina Clarke is a Vancouver- and Toronto-based journalist who writes about relationships, health, technology and social trends. You can find her on Twitter at @KatrinaAClarke.