What Research Says About Period Syncing

Menstruation | | Helena Lorimer
5 min read

Isn’t it strange when you and your best friend are on your period at the same time? Strange, but also kind of cool in a weird way? As if you’ve formed some sort of special bond with each other.

Or how about when your entire squad is on their period? Crazy! There has to be some logic behind this, right? Is it period syncing? Is it a coincidence? Who knows! 

Well, actually: professionals and researchers know! And now you will too! 

What Is Period Syncing?

Period syncing is the idea that when women live in close proximity to each other, or they spend a lot of time together, the timing of their monthly period will start to “sync up” or align. 

The first evidence to suggest that period syncing could be a “real thing” was in 1971 in a paper called the “McClintock study”. This study was named after Martha McClintock, a researcher at Harvard University.

The McClintock Study

McClintock began researching the topic by looking at the menstrual cycle of a small number of women aged 17-22 who lived in the college dorms. 

She divided the women into two groups: those within close proximity of each other, such as best friends or roommates, and those who had no relationship with a select group of women. 

After six months, she found that those who lived within close proximity of each other started their period within 3 to 7 days apart, whereas those who weren’t in close proximity started their period with 5 to 15 days apart. 

But, given the small scale of this study, and the fact that it is outdated, are the results still valid or reliable? Well, not really, according to other researchers.

Results of Other Studies that Researched Period Syncing 

After the McClintock study, the topic quickly gained traction and other researchers got to work. 

These are some of the results:

  • 1993: a study showed that 29 same-sex couples did not experience syncing.
  • 1995: a study showed that pairs of close friends who did not live together did not experience syncing.
  • 2017: the app Clue partnered with Oxford University to analyze 360 pairs of women who were close friends. After 3 cycles, 273 of the pairs noticed an even bigger difference in their period start date as opposed to the beginning of the study. 

The last study actually shows the opposite to be true and that period syncing is not a thing.

But, before you believe these studies to be 100% valid, it’s important to note that they were not peer-reviewed by medical experts. That means that the results have not been signed off as conclusive. 

In other words, more research needs to be done in order to finally conclude that period syncing is real. But, as it stands, it’s strongly suggested that period syncing is not real.

A Theory on Period Syncing

What is it that leads some to believe in period syncing? Why does it actually happen? 

Well, there’s a theory that suggests that, if period syncing is real, it has to do with the release of pheromones when women are in close proximity to each other. 

Pheromones are chemicals that, when released, influence the behavior of others. 

But this theory doesn’t really have much value today because of modern day habits.

“Pheromones are largely emitted from the armpits and the groin, but think about the habits of modern-day society. Maybe there’s something there, but if you’re washing it all off when you shower, the pheromones aren’t going to have much effect,” according to Dr. Stasia Jhaveri, ob/gyn.

Period Synching: Is It Merely a Question of Time & Mathematics?

Up until now, we’ve not found any reliable data to prove that period syncing is real. In fact, a lot of the studies indicate that it isn’t. 

But is there one possible explanation as to why you may feel as though your period is synced with your friends or roommates? Indeed. And it all has to do with time and mathematics. 

You see, over time, a woman who has a 3-week cycle and another who has a 5-week cycle will eventually see their periods overlap. 

It’s anecdotal that you’re likely to remember the times you had bad cramps at the same time your roommate did more than the times you didn’t,” says Dr. Jhaveri. “You will overlap and diverge because women have different cycle lengths.”

This makes sense.

And so, because there’s not enough evidence to prove that cycle syncing is real, what are some factors that actually do influence your period?

Factors that Influence Your Period

  • Being on the birth control pill: when you are on the birth control pill, it alters the levels of sex hormones in your body which, in turn, can control if or when you have a period.
  • Extreme stress: if you are extremely stressed, your body will release the stress hormone called cortisol. When you have high levels of cortisol, it could stop ovulation in its tracks, affecting if and when you have your period.
  • Chronic illness: a number of chronic illnesses such as diabetes, cystic fibrosis, renal disease, and inflammatory bowel disease can affect one’s menstrual cycle.
  • Anorexia/bulimia: repeated purging and binging can cause your period to become irregular or even stop it completely. 
  • Exercise: if you’re exercising excessively, it could cause you to miss your period or have it stop completely because of the change in hormones that extreme exercise can bring. 

So, while the idea behind period syncing is quite endearing, truth be told, it’s impossible to prove that it actually happens. In actuality, there are more studies that say it doesn’t than studies that say it does. 

Granted, more research could be done on the matter. But for now, sorry ladies: period syncing isn’t real. It’s merely a myth that could be fun to believe in. 

And hey, you’re not hurting anyone if you find comfort in being on your period at the same time as your bestie!

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