Period syncing describes a popular belief that women who live together or spend a lot of time together begin menstruating on the same day every month.

Period syncing is also known as “menstrual synchrony” and “the McClintock effect.” It’s based on the theory that when you come in physical contact with another person who menstruates, your pheromones influence each other so that eventually, your monthly cycles line up.

Some women even swear that certain “alpha females” can be the determining factor when entire groups of women experience ovulation and menstruation.

Anecdotally, people who menstruate accept that period syncing is a real thing that occurs. But the medical literature doesn’t have a solid case to prove that it happens. Keep reading to find out what we know about menstruation cycles syncing up.

The idea of period syncing has been passed down from mothers to their daughters and discussed in dorms and women’s restrooms for centuries. But the scientific community started to take the idea seriously when a researcher named Martha McClintock conducted a study of 135 college women living in a dorm together to see if their menstrual cycles aligned.

The study didn’t test other cycle factors, like when the women ovulated, but it did track when the women’s monthly bleeding began. McClintock concluded that the women’s periods were, indeed, syncing up. After that, period syncing was referred to as “the McClintock effect.”

With the invention of period tracking apps that store digital records of women’s cycles, there’s a lot more data available now to understand if period syncing is real. And the new research doesn’t support McClintock’s original conclusion.

In 2006, a of the literature made the assertion that “women do not sync their menstrual cycles.” This study collected data from 186 women living in groups in a dorm in China. Any period syncing that appeared to occur, the study concluded, was within the realm of mathematical coincidence.

A large study conducted by Oxford University and the period tracking app company Clue was the biggest blow yet to the theory of period syncing. Data from over 1,500 people demonstrated that it’s unlikely that women can disrupt each other’s menstrual cycles by being in close proximity to one another.

A much smaller keeps the idea of period syncing alive by pointing out that 44 percent of participants that were living with other women experienced period synchrony. Period symptoms like menstrual migraine were also more common in women living together. This would indicate that women might influence each other’s periods in ways beyond the timing of their menstruation.

The word “menstruation” is a combination of Latin and Greek words meaning “moon” and “month.” People have long believed that women’s fertility rhythms were related to the lunar cycle. And there’s some research to suggest that your period is connected to or somewhat syncs with the moon’s phases.

In one older study from 1986, of participants experienced period bleeding during the new moon phase. If this data set of 826 women held for the entire population, it would indicate that 1 in 4 women have their period during the new moon phase. However, a more recent study conducted in 2013 suggested .

The truth is, we might never nail down how real the phenomenon of period syncing is, for a few reasons.

Period syncing is controversial because we don’t know for sure if the pheromones on which the theory hinges can influence when your period starts.

Pheromones are chemical signals that we send to the other humans around us. They signify attraction, fertility, and sexual arousal, among other things. But can the pheromones from one woman signal to another that menstruation should take place? We don’t know.

Period syncing is also difficult to prove because of the logistics of women’s period cycles. While the standard menstrual cycle lasts for 28 days — beginning with 5 to 7 days of your “period” during which your uterus sheds and you experience bleeding — lots of people don’t experience periods that way.

Cycle lengths up to 40 days are still within the realm of what’s “normal.” Some women have shorter cycles with only two or three days of bleeding. That makes what we think of as “period syncing” a subjective metric that depends on how we define “syncing up.”

Menstrual synchrony might often appear due to the laws of probability more than anything else. If you have your period for one week out of the month, and you live with three other women, odds are at least two of you will be having your period at the same time. This probability complicates research into period syncing.

As with many women’s health issues, menstrual synchrony deserves more attention and research, despite how difficult it may be to prove or disprove. Until then, period syncing will probably continue to live on as an anecdotally proven belief about women’s periods.

As humans, it’s natural to connect our physical experiences with our emotional ones, and having a period that “syncs” with a family member or close friend adds another layer to our relationships. However, it’s important to note that having a period that’s “out of sync” with the women you live with doesn’t mean anything is irregular or wrong with your cycle or your relationships.