There’s a specific scene in the 2011 romantic comedy “No Strings Attached” (not to be confused with the 2011 romantic comedy “Friends with Benefits,” which is exactly the same movie but with an unnecessary flash mob) that irks me every time.
The three medical student roommates, played by Natalie Portman, Mindy Kaling, and Greta Gerwig, sulk around their chic (huge!) apartment, clutching hot water bottles to their abdomens, downing Pamprin, and exclaiming things like, “It’s like a crime scene in my pants!”
Then Ashton Kutcher enters the room. “Oh, I understand what’s going on!” he says. “You’re all on the same cycle! Your uterine walls will be shedding for the next three to five days.” It’s all just a lot to take in.
The scene may have some troubling dialogue, but is the central theme of “being on the same cycle” solid? It seems like just about every tight-knit menstruating friend group has a story about being synced up. Even Carrie, Miranda, and Charlotte cycled together (Samantha was a little delayed that month).
According to an oft-cited 1971 paper from researcher Martha McClintock, titled “Menstrual Synchrony and Suppression,” 135 cohabitating college women did begin to experience synchronized periods. But the much-hyped findings were merely observational — no other cycle factors, like ovulation, were studied. Nevertheless, people began referring to the concept of period syncing as “the McClintock effect.”
So what’s the deal? Do women who live, work, or hang out together really bleed together? Well, it depends on who you ask — and whether you subscribe to one of three major theories on the subject.
According to a 1998 paper from McClintock and Kathleen Stern, chemical signals may be at the root of menstrual syncing. You’ve probably heard of pheromones in the context of sexual attraction — they’re chemicals humans and animals secrete that can affect the behavior of other members of the same species (like arousal).
McClintock and Stern studied underarm compounds of female participants at various points in their menstrual cycles and thought the release of pheromones could manipulate ovulation in other females (aka alpha females determining the cycles of others).
Not so fast, though. In 2006, researchers Zhengwei Yang and Jeffrey C. Schank clapped back with their aptly titled paper “Women Do Not Synchronize Their Menstrual Cycles.” Its subsequent “supporting” evidence stated that “the original results (McClintock 1971) were flawed and at the level of chance.”
As for the pheromone theory, Yang and Schank collected data from 29 groups of women living in dorms of a Chinese university for more than a year. Four to eight women lived in each room, “which should be ideal conditions for synchronization if pheromones are the mechanism of synchrony.”
But the researchers found — in the largest study ever undertaken on the subject — that “synchrony did not occur when women lived together for a year or more, and that menstrual cycle onset clusterings were neither stable nor occurred more than expected by chance.”
So no, pheromones are not responsible for your friend group’s seemingly synchronized PMS experiences. Could something else be behind menstrual syncing?
It’s hard to deny that the moon seems like a fitting dictator of female reproductive energy — after all, the words “menstruation” and “menses” even come from Latin and Greek words for month (mensis) and moon (mene). And over the last century, many studies have found interesting moon-related menstrual tidbits, like a 1986 paper that found people were more likely to menstruate during a new moon.
But according to a Clue analysis of 7.5 million cycles — the largest ever done — the menstrual cycle and lunar cycle aren’t actually in sync. Clue researchers found that the global average menstrual cycle length is 29 days (although anywhere from 21 to 35 days is considered normal) and the lunar cycle lasts 29.5 days.
Sounds too freaky to be a coincidence, right?
Except that, according to Clue’s Dr. Marija Vlajic Wheeler, the analysis showed there is no indication that menstruation predominantly starts during the new moon, as is often claimed.
If you and your friends or co-workers seem to notice yourselves syncing up with the moon, researchers have some other possible explanations. For one thing, moonlight itself may affect biological processes — particularly in the absence of artificial light. Other researchers have suggested that lunar electromagnetic radiation may affect menstrual cycles, but not everyone’s convinced.
It’s important to remember that the moon and its relationship to menstruation — whether spiritual, metaphorical, or otherwise — can hold a lot of personal meaning for some people. But according to scientists, relying on the moon as a foolproof indicator of menstruation or even fertility isn’t recommended.
Here’s the real deal, according to current research: Period syncing is hard to prove because menstrual cycles aren’t uniform across individuals and populations. Sure, 28 days is standard, but there’s a ton of variation that’s still well within the boundaries of “standard.”
Think about it: If you have a 27-day cycle and your friend has a 32-day cycle, chances are you’ll eventually have periods that overlap. It may seem like you’re synced up with friends, co-workers, and family just because of the basic laws of probability.
Statistically speaking, at some point, it’s more likely than not you’ll be bleeding at the same time as someone you know. There’s no scientific evidence that that overlap is “synchrony.” It’s probably just chance.
The reason so many women (and researchers) are likely fascinated by the synchrony concept is that, well, misery loves company. It can feel super validating and comforting to know the women closest to you are going through the exact same thing you are — especially when that thing involves blood, cramps, bloating, and possibly more.
And it can feel extremely isolating to be left out of a group that seems in sync (see: earlier reference to nearly pre-menopausal Samantha in “Sex and the City”). But the science should help alleviate those concerns — menstrual cycles are variable, complicated, and not likely to be synchronized for any other reason than chance.
So if you’re bleeding before, after, or on a totally different schedule than your pals, that’s totally OK and absolutely your normal.
While it might be a cute concept for rom-com purposes, period syncing just hasn’t been proven to be real. Despite early research indicating that women who live, work, or hang out together for extended periods of time could be influencing each other’s cycles through chemicals, or that the moon could be having a larger effect on all menstruating people, current studies say otherwise.
Period syncing is more likely a result of statistical probability than of anything else — if you bleed once every month, chances are someone else in your life will be bleeding around that same time.
The biggest benefit of clinging to the menstrual syncing theories is potentially the feeling of being validated and understood by others going through the same experience. But even if syncing may be a myth, connectedness and community around periods can and should still be cultivated.
Rather than hoping for a friend group that’s perfectly synced up, seek out ways to bond with others by talking about symptoms, sharing stories, and just being supportive of one another. No matter what your menstrual schedule is, creating a lifestyle that helps you feel healthy, happy, and totally understood can make your periods a lot more pleasant.
Michelle Konstantinovsky is a San Francisco-based journalist, marketing specialist, ghostwriter, and UC Berkeley Graduate School of Journalism alumna. She’s written extensively on health, body image, entertainment, lifestyle, design, and tech.