Ever lounged on the couch, eaten ice cream, and fought over the heating pad with your roommate? Or perhaps you never need to worry about having ibuprofen or your favorite essential oil on hand, because you know your coworker will also have some around a certain time of the month.
According to a 1999 study, 80 percent of women believe in period syncing and 70 percent reported that they enjoy when it happens to them. But as much as we want to believe that our periods do sync up (there’s something about the camaraderie it creates), we can’t help but wonder if it’s actually true. Is there scientific evidence or data that supports cycle synchronization when women spend a lot of time together?
For over 40 years, researchers and doctors have been trying to figure out the answer, and they’ve had little luck—until recently. Here’s what the scientific community has learned over the years and what they discovered in 2017.
The 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 the school year, there was an increase in period synchronization for roommates and close friends, but not among random pairings of women.
McClintock hypothesized that this was due to time spent together, eating meals together, experiencing stress together, or even a theory called the “alpha uterus” (seriously). According to the study, an alpha uterus has a “strong hormonal pull that causes other cycles around it to menstruate in unison.” She proposed that when a woman becomes aware of another woman’s period (by hearing her mention it or seeing her carry a pad to the bathroom), it might cause them to start her period. The study suggested that these things can cause women’s pheromones to communicate with each other due to physical closeness. This, in turn, triggers 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. 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" that human pheromones could manipulate the timing of ovulation. Beverly Strassman, professor of anthropology at the University of Michigan, used their assertion to inform her 1999 paper in Human Reproduction, arguing the opposite:
"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 a 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
This past year, the period and fertility tracking app Clue, in collaboration with a doctor from Oxford University, tried to find the answer by taking a more data-driven approach.
The data science team surveyed Clue users and asked if they thought their cycle had been syncing with another woman’s, who also uses the app. Then, they asked the users what type of relationship they had with this other person (friends, siblings, partners, roommates, coworkers, etc.), if they live together, and if they’re on hormonal birth control.
The Clue team received over 1,500 responses and narrowed it down to 360 pairs of users whose cycles occurred during a similar time period. From there, Clue tracked three consecutive cycles for each pair. Here’s what they found:
“273 pairs (76 percent of the sample) actually had a larger difference in cycle start dates at the end of the study than at the beginning of the study. Only 79 of the pairs behaved in the opposite way, with the gap between cycle start dates getting closer during the study. For all 360 pairs, the average difference at the beginning was 10 days, and 38 days at the end.”
Based on this study, your cycle is actually more likely to get out of sync than in sync with another woman’s period.
The Clue study is the most data-driven and has the largest sample size compared to the others, indicates that it’s unlikely that women’s cycles sync. But it’s important to keep in mind that this was still a relatively small study. There’s more research to be done on this topic and in women’s health in general—something that’s unfortunately been overlooked for decades—until we know for certain.
Even though the research says women’s cycles don’t converge, that doesn’t mean you can’t enjoy the feeling of comfort, connection, or sisterhood that comes along with having your period at the same time as someone else. So, keep knowingly nodding when a sister asks to borrow a tampon. While your cycles may not be biologically syncing, you’re still going through the same thing.