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Why Do Periods Sync with Friends?Most menstruators tend to believe in the synchronized menstrual cycles phenomenon because their own cycles have at one point lined up with a parent, partner, sibling, roommate, or close friend living in close proximity. But what are the chances that there is truly scientific evidence?
The odds are actually in its favor. If you consider that on average, we menstruate for 25% of the month and that all menstruators have cycles of different lengths, it seems very possible that any two people will eventually see their menstrual phases line up.
Let’s do the math. If the average menstrual cycle lasts 28 days, the farthest apart two cycles could be is 14 days, and because cycle lengths tend to fluctuate by a few days, these two cycles will only get closer together.
The question then becomes, do periods sync with friends because they are having an actual impact on our hormonal functioning or just by a matter of chance? Fortunately, we are not the only ones asking this question, and here's the research that’s been done.
Do Periods Sync Up with Friends?While (spoiler alert!) modern research has concluded that syncing is solely mathematical and based on probability and has nothing to do with one menstruator affecting another. But it took a long time to get there. Many different studies have been conducted since the 1970s, with one that notably convinced us that it was scientifically based.
Martha McClintock was the first researcher to attempt to answer this question. In 1971, McClintock studied 135 women who lived in the same college dorm. The study only tracked the beginning of each of their periods, and based on this data alone, McClintock concluded that their cycles were getting closer together each month. Her research concluded that social interactions between menstruators could transmit pheromones and were therefore causing those who interact frequently to sync up.
This study named it “the McClintock effect”, which gained popular recognition. But it wasn’t until years later, when critics began to point out the flaws in her scientific process and compared her findings to other studies, that this theory was then proven wrong.
Proven FictionIn 2006, a study tracked 186 college students living in dorms in China. Though there were many times that the women’s menstrual phases synced up, the researchers couldn’t conclude that there wasn’t any one cycle was influencing another. Their conclusion for this syncing was instead that mathematically their monthly periods were bound to sync up eventually.
Another study out of Oxford University took data from over 1,500 menstruators and also showed that menstruation can occur simultaneously for women who live in proximity but there is no reason to believe causation.
Migraines & Period SyncingInterestingly, one thing that does show synchronicity is the prevalence of migraines for menstruators living together. According to a study done in 2017, menstruators living together experienced more migraines. This finding was not shown to be related to contraceptive use, stress, or sleep deprivation.
Though this study was small, and more research is required to better understand why this occurs, it does suggest the potential for other ways menstruators may unknowingly influence one another.
Ultimately, the idea of period syncing remains a myth—but mathematically it still often occurs! So, if syncing up with your friends was something that made getting your period a little more fun, a little less stressful, or something to bond over, math is still on your side. Knowing this, try comparing period tracking apps to work out approximately when you will sync up, so you can plan for the ultimate self-care night in!
When you do sync up, give yourself and your fellow menstruators some extra space to relax and some time to listen to your bodies as you both go navigate the menstrual phase. Take this opportunity to finally switch to reusable period products like the DIVA Cup or DIVA Disc together and encourage each other to maintain good period care habits like drinking plenty of water, getting a good night’s sleep, and taking care of your hardworking, menstruating body!
- Ferreira KS;Guilherme G;Faria VR;Borges LM;Uchiyama AA; (n.d.). Women living together have a higher frequency of menstrual migraine. Headache. https://pubmed.ncbi.nlm.nih.gov/27861827/
- MAPSS. (n.d.). https://mapss.uchicago.edu/directory/martha-mcclintock
- Strassmann, B. I. (1999, March 1). Academic.oup.com. https://academic.oup.com/humrep/article/14/3/579/632869
- Watson, K. (2019, January 23). Is period syncing a real thing? why women’s periods may sync up. Healthline. https://www.healthline.com/health/womens-health/period-syncing#obstacles-to-proving