Do Periods Sync? Learn the Science Behind It

Is it true that people who spend a lot of time together get their periods at the same time? Scroll down for the answers to your burning questions.

You may have heard of period syncing, also known as “menstrual synchrony.” Period syncing refers to the phenomenon that people who menstruate and live together, or spend a lot of time together, or come into physical contact with each other, seem to get their menstrual cycles at the same time.

The hypothesis is that their pheremones influence and affect each other and will, therefore, make their periods come (and go) at the same time. Some people even think that there can even be an “alpha” in a group that sets the cycle that everyone else syncs to.

Does this sound a bit weird to you? Or even unrealistic? Is period syncing for real, or is it just an urban legend you heard from a friend’s older sibling when you were a kid? Most people who menstruate accept syncing as a real thing that happens. Let’s get into it and take a peek at the science behind period syncing, and see if it’s even something that goes on.

three women in a school locker room

Wait, what are pheromones?

Pheromones are chemical substances that are released from the body in fluids such as pee and sweat. Animals will secrete pheromones to send messages to one another (within the same species), triggering a response. Essentially, pheromones function as a form of secret, invisible, and hidden communication.

The jury is still out on whether or not humans make pheromones, and whether or not they play a role in things like attraction, reproduction, and yes, menstrual cycle syncing. 


What is period syncing?

The truth is, there isn’t really any concrete evidence to prove that period syncing even exists to begin with. The phenomenon first appeared in a science paper in 1971. It was studied by a researcher and psychologist named Martha McClintock, who spoke to 135 college women living in a dorm together. McClintock didn’t study all cycle factors, such as ovulation, but she did track when their monthly bleeding actually began.

From this study, McClintock was able to conclude that their monthly period cycles did sync up, and she chalked this up to pheromones. Cycle syncing began to be known as the “McClintock Effect.”

two women taking a selfie

So then it’s real?

Despite this supposed proof that period syncing is real, there’s been a wave of evidence since 1971 suggesting the opposite is true. For example, a study following 186 women living in a dorm in China determined that people who menstruate don’t sync their periods at all. 

There's also the myth that our periods are synced with the lunar cycles, since the menstrual cycle length is closely linked to the moon phases. However, there is not much evidence to support this theory.

So what is the truth?

Honestly? The truth is that there’s simply not enough science and research on period cycle syncing to prove that it’s real or not real either way. Many researchers and scientists these days tend to lean more towards disbelief.

Marija Vlajic, a scientist for the period tracker app Clue, says that “[i]t’s very unlikely that cycle syncing is a real phenomenon. Menstrual syncing amongst the sample we had did not exist.


Should my teen tell their friends when they get their period?

They can tell whoever they want whatever they want, as long as they're comfortable. It’s great to open up and talk about all things periods with supportive friends when they're ready, whether they want to open up about period symptoms or cycle length. It always helps to find comfort in community and solidarity amongst peers when going through big life changes like this. Periods are something they'll be experiencing for a large part of their life, so removing the awkwardness around them is crucial.

one woman whispering a secret to another

Your teen can talk to friends about tampons, cycle lengths, whether or not they’ve gotten their first periods, any stories they've heard from older siblings or you, their parents. The topics of conversation are endless, and can serve to bring them closer together with their friends —whether their menstrual cycles are synced or not. And they're feeling worried about potentially experiencing leaks or cramps at school, check out the linked blog posts to figure out what to do when surprise periods strike (hey period underwear, we're looking at you!).


So, what did we learn? Period cycle syncing may or may be a thing, but the science and menstrual cycle data seems to lean more to the side of non-existence. While Martha McClintock’s original 1971 paper offered a resounding “Yes!,” reviews of these studies and the phenomenon in 2006 and 2013 disproved the idea, while a smaller 2017 study suggests that menstruating people who live together might affect each other in more ways than just period timing, such as via menstrual migraines and more.

For now, the science and the answer is still hazy. Perhaps your teen will experience period syncing with friends and family at some point in their life, or perhaps it’ll be just a coincidence. Either way, now you know where the idea came from. 

Oh, and just in case—make sure you take a peek at some of our Period Underwear. It’s a great solution to prevent leaks, no matter if your teen is synced up or not.

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