Is Period Syncing Real?

July 15, 2020

Period syncing, the theory that women who live together or see each other a lot end up menstruating at the same time each month, has been around for quite some time. This phenomenon is sometimes referred to as “menstrual synchrony” or “the McClintock effect” (thanks to one of the first studies conducted on the topic by a doctor by the name of Martha McClintock). There are a lot of myths around this widely held belief, so today, we’re getting to the bottom of it. Is period syncing real or a myth? And if it is real, how exactly does it happen? Keep reading for the answers to these questions and more!

What is period syncing?

As mentioned above, period syncing all began with Martha McClintock, a Harvard doctor who, in 1971, studied a group of 135 women living in the same college dormitory to see if their menstrual cycles aligned. The study only tracked when each woman’s monthly bleeding began. The result? Over the course of the school year, McClintock found that many of the women’s periods did in fact sync up. However, it’s worth noting that there was an increase in period synchronization among close friends or roommates, not just two randomly paired women in the dorm. In her findings, McClintock suggested that when women have regular physical contact with each other, their pheromones influence each other until the point that their menstrual cycles eventually align. Overall, period syncing as we know it was born out of the McClintock study. But as we all know, one study doesn’t make something fact, and so the question remains, is period syncing real or a myth?

Is period syncing a real thing?

Studies have found that over 80 percent of women believe in period syncing, with 70 percent even reporting that they enjoy it when it happens to them (there’s nothing like a shared menstrual cycle to bond two women!). However, despite these overwhelming statistics, the more recent science is mixed on whether or not period syncing is real. Since McClintock’s 1971 study, there have been multiple others conducted on everyone from female roommates and best friends to lesbian couples and yet there is still no consensus. Some studies have even reached the opposite conclusion to McClintock. For example, a similar 2006 study collected period data from 186 women living in a dorm in China and the results showed that the women’s periods didn’t sync up at all. The limited period syncing that did occur was small enough for the researchers to chalk it up to mathematical coincidence.

The largest study ever done on this topic was conducted by Oxford University, in conjunction with the period tracking app company, Clue in 2017. (Side note: period tracking apps have been hugely helping in researching this phenomenon.) As part of the study, Oxford assessed data from over 1,500 women, and found only 360 pairs of users whose cycles occurred during a similar time period. Of these 360 pairs, which were then tracked for three consecutive menstrual cycles by Clue, 273 of them (76 percent) ended up with a larger difference in cycle start dates at the end of the study than at the beginning. So far, the Oxford University/Clue study has been the biggest blow to the theory of period synchronization to date. But the reality is that at this point in time, there is simply not enough scientific evidence to unequivocally determine whether period syncing is for real or not.

Why is the research on period syncing so mixed?

There are many obstacles that make period syncing extremely hard to prove. First, period syncing rests on the notion that women’s pheromones influence each other. This in and of itself is a controversial statement within the scientific community as there is far from enough research to determine whether pheromones actually have the power to influence when someone’s period starts. Further, period syncing is hard to prove because there is no “one size fits all” menstrual cycle. Even though the average menstrual cycle lasts for 28 days, this is not the case for all women. Cycles up to 40 days are still considered “normal,” while the number of days that your actual period lasts can range between two to seven. This reality makes period syncing a logistical nightmare for researchers. It’s also worth noting that due to the length of many women’s cycles, period syncing might often occur due to the laws of probability more than any other factor. This element complicates research even further. One final but important point: Frankly, women’s health has been overlooked throughout history, which is why up to now, so little research has been done on this topic (not to mention many other, much more serious women’s health issues). Thankfully, owing to the onset of period tracking apps that are now more popular than ever, it may be possible to learn more about this in the years to come.