This article was last updated April 2021.
Have you ever lounged on the couch, eaten ice cream, and fought over the heating pad with your roommate? Or perhaps you’re never worried about having ibuprofen on hand because you know your coworker will be prepared around the same time of the month?
According to a 1999 study, 84% of participants reported knowing about menstrual period syncing and 70% said they had personal experiences with the phenomenon. But as much as some of us may want to believe our periods sync up and celebrate that feeling of camaraderie, that doesn't been there's any solid scientific evidence that it happens.
For over 40 years, researchers and doctors have tried to answer that question with mixed findings — but these days, the science community is fairly confident that period syncing isn't a thing. Keep reading to understand how they came to that conclusion, what we've learned over the years, and where more research is needed.
First: What is period syncing?
The textbook term for synced periods is menstrual synchrony, also known as the McClintock Effect (we’ll explain where that name comes from in just a bit). When we talk about periods syncing, we’re referencing the idea that being in frequent close proximity with another menstruating person (and that person’s pheromones) affects your biology and prompts your period to start sooner or later than what’s normal for you.
The period syncing study that started it all
In 1971, Harvard doctor Martha McClintock studied a group of 135 women living in a college dorm. Their research found that over the course of a school year, period synchronization increased among roommates and close friends, but not among random pairings of women who hadn’t been spending time together.
McClintock hypothesized that synchronization was the result of spending time together, eating meals together, or experiencing stress together — and even suggested a theory called the “alpha uterus” (seriously).
According to McClintock’s study, an alpha uterus has a “strong hormonal pull that causes other cycles around it to menstruate in unison.” She proposed that when someone becomes aware of another person’s period (by hearing it mentioned or seeing someone carry a pad to the bathroom), then their period might start, too. This theory is known as the McClintock Effect, which is the idea that women’s pheromones communicate with each other due to physical closeness, triggering cycle syncing.
A series of mixed findings
Since 1971, multiple studies have been conducted on female roommates, best friends, lesbian couples, and even animals to see if menstrual cycles actually converge while spending time together. But nearly all of these studies have mixed findings and two in particular fail to replicate the 1971 study’s results. A 2006 study, for example, collected period data from 186 Chinese women living in a college dorm together, and the results showed that the women’s cycles didn’t sync up at all.
In a 1998 study, McClintock, along with Kathleen Stern, announced that they had "definitive evidence" of the McClintock Effect, proving that human pheromones could manipulate the timing of ovulation. But Beverly Strassman, professor of anthropology at the University of Michigan, used their assertion to inform her 1999 paper in Human Reproduction, arguing the opposite while analyzing cycle length:
"If the menstrual cycle is 28 days long, then 14 days apart would be maximal asynchrony. By chance alone, one would expect two women to be 7 days apart (half of 14 days). Given that menstruation can last 5 days, overlapping periods are a common occurrence. That women synchronize to each other, however, is a myth."
The latest? Cycles don’t converge — they actually diverge
Subsequent studies from the last couple of decades continue to suggest that menstrual syncing with people around you is not a likely occurrence.
First, the 2006 study of 186 college students we mentioned earlier demonstrated that:
- "Women living in groups did not synchronize their cycles," according to the study's authors.
- Moreover, these researchers noted that any group synchrony that occurred was “at the level of chance,” and variability of period cycles over time inevitably resulted in “convergences and subsequent divergences of cycle onsets.”
- This could make someone assume they're syncing with other people's periods when it’s actually just a coincidence when cycles line up.
Then, 11 years later, researchers from a period-tracking app alongside Oxford University collected digital data from app users in a 2017 pilot study to observe changes in cycle length and occurrences of period syncing versus diverging. (FYI, this pilot study hasn't been published anywhere but the app's blog so far.)
Researchers first asked app users if they thought their menstrual cycle synced with another user’s cycle, as well as their relationship with this other person (i.e. friends, siblings, partners, roommates, coworkers, etc.), if they lived together, and if they were on hormonal birth control. After receiving over 1,500 responses and narrowing it down to 360 pairs of users whose cycles occurred at similar times, they tracked three consecutive cycles for each pair. Here are their most significant findings:
- At the end of the pilot study, they found that 273 pairs (which comprised 76% of the sample) actually had a bigger difference in their cycle start dates when compared to data from the beginning of the pilot study.
- Only 79 pairs saw their cycle start dates get closer.
- The average difference in cycle start dates across all 360 pairs was 10 days at the beginning of the pilot study and 38 by the end.
- Based on this preliminary data, your cycle is actually more likely to get out of sync than in sync with someone else's cycle.
Do periods sync for other reasons?
As the Cleveland Clinic tells us, period syncing often comes down to a simple matter of time, rather than any kind of biological phenomenon. For example, if you live with another menstruating person for at least one year, your cycle length of four weeks and your roommate’s cycle length of five weeks “will eventually… coincide and diverge again.”
If you do experience late or surprise periods or your cycle length is somehow altered, there are a few potential causes to consider before attributing it to a friend you’ve been spending time with. Hormonal birth control, stress, medications, health conditions, and disordered eating can all interrupt your usual cycle.
Our take on the question "do women's periods sync?"
While the study from the app team and Oxford doctor never moved past the "pilot" phase and into a peer-reviewed journal, the sample size was the largest compared to past studies. More research needs to be done on this topic (and in women’s health generally, a topic that’s unfortunately been overlooked for decades) until we know for certain.
(P.S. If you're interested in moving fertility and reproductive health research forward, check out what we're working on at Modern Fertility and get in on the action.)
Even though more recent research says people's menstrual cycles don’t converge, that doesn’t mean you can’t enjoy the feeling of comfort and connection that accompanies having your period at the same time as someone else. So, feel free to keep on knowingly nodding when friends or family members ask to borrow a tampon. While your cycles may not be biologically syncing, you’re still going through the same thing.