Period Bluetooth

“You’ve also gotten your period today?” Almost every woman can tell the same story- that her menstrual cycle got synced up with a close friend or a family member. This phenomenon, known as "menstrual synchrony," has been the subject of heated discussions, whispered dorm-room conspiracies, and even a bit of scientific inquiry. Some women even believe that certain “alpha females” can be the determining factor when entire groups of women experience ovulation and menstruation.

The idea of menstrual cycles having a sort of ‘bluetooth effect’ gained scientific legitimacy in 1971 when psychologist Martha McClintock published a study in the journal Nature. McClintock observed 135 women living in a college dorm and found that their menstrual cycles appeared to align over time. 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.” She hypothesized that pheromones—chemical signals animals release to communicate with others of their kind—might be responsible for this phenomenon.

Among animals a lot of evidence exists to support the power of pheromones to impact and change hormonal behaviors. The pheromone hypothesis remains tantalizing, even if it’s largely unproven. Could humans be sending out subconscious “sync up” signals to those around them? It might be true however there’s hardly any solid evidence to confirm this. Human pheromones are very tricky to study, and any effects they have on menstrual cycles remain speculative at best.

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 new study and review 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 study in 2017 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. In one older study from 1986, over 28 percent 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 no connection.

The truth is, we might never figure out 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.

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.

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.
Science may not confirm a chemical or hormonal reason to explain why your period might match up with a roommate or close friend, but there is a mathematical explanation: It’s simply a matter of time. Menstrual cycles are anything but clockwork. The average cycle is 28 days, but it can range from 21 to 35 days. With so much variation, cycles may appear to overlap purely by chance without any underlying synchronization.

While menstrual synchrony might not hold up under scientific scrutiny, the rumor persists because it taps into something real: the deep connections we form with the people in our lives. Science may eventually settle the question for good, but until then, we’ll keep swapping stories, sharing laughs, and sticking together—whether or not our cycles do the same.

Bibliography 

https://health.clevelandclinic.org/myth-truth-period-really-sync-close-friends

https://www.healthline.com/health/womens-health/period-syncing

https://periodeducationproject.org/2021/11/23/is-period-syncing-real/

https://www.bbc.com/news/magazine-37256161

https://www.businessinsider.com/guides/health/reproductive-health/why-do-womens-periods-sync

https://www.controlcramps.com/blog/a-look-at-the-science-why-do-womens-menstrual-cycles-sync-up/

https://www.washingtonpost.com/wellness/2024/04/22/women-menstruation-periods-synchrony/