Period synching involves the idea that people who menstruate and spend a lot of time with each other, may find that their periods begin to start at the same time. A study conducted in 1999 revealed that 70% of participants have had personal experience with period synching. Many menstruating people will also attest to this, however, science and research does not (yet) seem to be able to back this. So what is the truth?
The idea of period synching (also known as menstrual synchrony) comes from a 1971 study by psychologist Martha McClintock. The research was based on studying a group of 135 women who lived in a college dorm. I must note that as is the case with most early studies, this collection of people–young women in college in the seventies–is exclusionary and underrepresenting of all peoples. Over the course of the study, it was found that period synchronization had increased among roommate and close friends, but not between random women who had not spent time together. McClintock theorized that it was this spending time together that resulted in the synchronization of periods. Synchronization may also be the result of what McClintock calls “the alpha uterus.” This alpha uterus has a “strong hormonal pull that causes other cycles around it to menstruate in unison.” This is further known as the “McClintock Effect” which revolves around the idea that it is fundamentally women’s pheromones communicating during physical closeness that triggers cycle synching. Pheromones can be defined as “substances which are secreted to the outside by an individual and received by a second individual of the same species” and play a key role in attraction.
Later on in 2006, a similar study was carried out. This study focused on 186 women who were living in groups in a dorm in China. They concluded that the cycles did not sync up at all, and that any group synchrony was merely a mathematical chance. Beverly Strassman, professor of anthropology, further discusses the idea of chance and menstrual synchrony: “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." An even larger study conducted by Oxford University in 2017 found that your cycle is far more likely to get out of sync than in sync with another person's cycle. This study, however, was only based on results in an app and has never been published on scientific groups, it has only been released in the app’s blog section.
Despite this contradicting evidence, other studies have still shown that pheromones may influence period’s in a way that goes beyond the timing of menstruation. A 2017 study not only found that 44% of participants that were living together experienced period synchrony, but that period symptoms such as migraines were far more common in women living together. These varying conclusions demonstrate just how hard it is to prove or disprove synchronicity in menstruation. The basic understanding that period synching is based on pheromones is already an unstable theory. Whilst we know pheromones play a key role in attraction, fertility, and sexual arousal, we do not know if pheromones can signal to another person that menstruation should take place. Differences in menstruation from one person to another can make proving period syncing a challenge. Although the standard menstrual cycle lasts for 28 days, with a period lasting between 5 and 7 days, many people do not experience periods in this standard matter. Some people experience a menstrual cycle that lasts for 40 days, and hence have longer periods. Others may only experience short cycles, with only 2 or 3 days of bleeding. Because of these vast differences, period synching can fall under a subjective metric that depends on how we define “syncing up.”
It is apparent that there is no scientific proof of period synchronization—alpha uterus’ and pheromones play no role in changes to the menstrual cycle. Things that can affect the timing of your period include: birth control pills, extreme stress, chronic illness, and eating disorders. If you do notice a sudden or serious change in your periods, it is best to seek medical advice and support. Menstruation as a whole needs more attention and research, and until that is achieved, the idea of period synching will simply live as an anecdotal belief. Note that having a period that is out of sync from other menstruating people you live with does not mean that anything is wrong with you, your cycle, or your relationship with them. If you do find that you and your friend or housemate have synched up, cool beans! Grab some of your favorite food, some hot water bottles and throw on a feel good movie—personally, I recommend 10 Things I Hate About You.
By Stephanie McCartney