Do Women's Periods Really Sync Up? The Truth About Menstrual Synchrony, Explained

At some point in your post-puberty life, you've probably encountered a moment when you and your friends realized your periods were synced up. One of you starts griping about your heavy flow, then another chimes in, saying their period just started as well, and next thing you know, all of you are sharing tampons and bragging about your amazing biological connection. In the medical community, this phenomenon is called "menstrual synchrony." But is it a real scientific phenomenon — or just a coincidence?

The first study ever study on the topic of menstrual synchrony was published in 1971 by a Harvard psychologist named Martha McClintock. She surveyed 135 women between the ages of 17 and 22, all of whom were living together in a college dorm. Her research found that these women's periods had synced up, and the study proposed that the pheromones and biochemicals secreted from each individual's body had been the cause. Called "Menstrual Synchrony and Suppression," it was a revolutionary paper that generated tremendous interest on an unprecedented topic.

Since then, countless researchers and doctors have come forward to further the conversation McClintock started. Some studies have found similar results; one in particular claimed that the closer the friendships, the more likely periods synced (although mother-daughter relationships didn't show the same synchrony). It's been suggested that ovarian-based pheromones from one woman — odorless compounds emitted from parts of the body such as the armpits — can cause a delay or acceleration of luteinizing hormones in another woman, which are responsible for the timing of their menstrual cycle. Of course, any kind of menstrual synchrony discovered would exist outside the realms of menstrual cycles dictated by hormonal birth control cycles.

However, there were plenty of critics of what McClintock and later similar studies proposed, who insisted that there wasn't a wide enough range of women tested to verify the theory, or that the observational period was too short. In 2013, a study that focused on ovulation rather than menstruation proved that women's ovulation windows didn't match up at all in the same pheromones-based way their periods do, leaving scientists no choice but to reject any evolutionary reasoning behind menstrual synchrony. Furthermore, women's cycles differ in length all the time — one month it will be 28 days, while the next it could be 32. There simply isn't enough consistency to say that the research done on menstrual synchrony leads us to a solid conclusion.

At this time, it's widely believed in the medical community that menstrual synchrony is more myth than reality. There simply isn't enough reliable evidence to prove that it biologically exists, since most of the studies that have been done on the topic have been flawed at best and completely inaccurate at worst.

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But maybe we've been asking the wrong question. Maybe it doesn't really matter that much whether menstrual synchrony is a total myth.

The journal Women's Reproductive Health recently published a study called "Demystifying Menstrual Synchrony: Women’s Subjective Beliefs About Bleeding in Tandem With Other Women," and it posits a different kind of question about the topic. Rather than grasping for scientific certainty, study author Breanne Fahs attempts to find out why women believe in menstrual synchrony. Is there a deeper reason behind our desire for it to be true? What purpose does menstrual synchrony serve in our minds?

There are very few studies that uncover what women themselves have to say about menstrual synchrony. A British study published in 1999 found that women feel overwhelmingly positive about their periods coordinating with the women around them. Similarly, just last year, a study of young Ethiopian girls showed that their belief in menstrual synchrony made them feel secure and united with their sisters in the face of cultural conflict. But Fahs wanted to delve deeper into women's thoughts on the matter.

Bustle spoke with Fahs about her work, who says this article is "looking at the ways that cultural stories about menstruation interact with science, often imperfectly." Fahs and her team of researchers gathered 20 adult women from a large metropolitan southwestern city. Minorities were intentionally overrepresented; 60 percent of the participants were white women and 40 percent were women of color. In individual interviews, the women shared their views on menstrual synchrony, whether they believed in it, and how this phenomenon relates to larger cultural views on menstruation.

95 percent of the women said it's definitely possible for women to have their periods at the same time. In fact, 90 percent said they have experienced this firsthand at some point in their life. Researchers found that women spoke positively about this period phenomenon because it was a tangible way for them to feel connected to other menstruating individuals, as well as to nature around them. "They want to feel solidarity with other women. They feel a closer connection to nature through their bodies," Fahs tells Bustle. Menstrual synchrony seems to hold an animal-like, mysterious quality to women, one that makes them feel somewhat alluring and magical.

Perhaps most importantly, Fahs' research even suggests that when women give life to the idea of menstrual synchrony, they reduce the period taboo and menstrual shame that society has placed on them. "Stigma can be lonely," Fahs says. "I think women get tired of carrying around the 'heaviness' of their menstrual status. It feels better to carry that together with other women." In other words, they take comfort in the fact that they don't have to face their periods alone.

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When you consider how menstrual synchrony is portrayed in the media, it's no wonder we get excited when we're bleeding at the same time our friends are. In TV shows and movies, men always speak of menstruating women as irrational, painstakingly emotional beings. For example, a 2012 episode of Modern Family called "Leap Day" portrayed Claire Dunphy and her two daughters as menstruating monsters who collectively bawled their eyes out at images of abandoned animals, poured orange juice into their cereal, and screamed hysterically at the simplest of questions. Husband and father Phil Dunphy is fearful for his safety, and he's terrified that these period-stricken women will ruin all the fun plans he's put together for Leap Day. Because a group of menstruating women is scarier than Hannibal Lecter, apparently.

From the time we experience our first period, women are the butt of these kinds of menstruation jokes. When we're bleeding, we're considered unreasonable and volatile; supposedly, we're not ourselves. But Fahs' study suggests that these unfair (and often cruel) stereotypes hold no power over us when we stand united alongside friends who are also menstruating. It's as if simply knowing we're not alone gives us the strength to overcome period taboo, even if only temporarily.

"Menstrual synchrony beliefs might be a way to fight back against this taboo and to be more open about bleeding," Fahs tells Bustle. She says wants to see "women transform menstruation from a shame-based experience, where they are silent and often feel 'gross' about their bodies, to one that feels more empowering." And she hopes to see our culture shed light on this change.

In the meantime, the next time you and your girlfriends discover you're simultaneously bleeding, don't even bother to question whether there's any scientific reason behind your menstrual connection. Just be happy you're not alone, and take Fahs' advice: rather than being grossed out by your period (like society tells you to be), choose to see your body as a site of knowledge and solidarity. Because it's pretty amazing what your vagina — and your friends' vaginas — can do.

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