For centuries, women’s periods have been shrouded in superstition, myths, and stigma. So it’s no surprise that certain beliefs about the menstrual cycle take hold — whether they’re based in science or not.
A leading example of this is period syncing. One study found that 90% of women believe their periods sync with their friends and roommates, even though the science backing the phenomena is mixed, at best (1, 4). The bottom line: menstrual syncing most likely is not a thing.
Here we unpack this myth and another popular belief about lunar cycles and menstruation. We explain why natural variability in the menstrual cycle sometimes makes your period seem synced with the cycle of your roommate or bestie.
Side note: Before we discuss the evidence, it’s worth noting that menstrual syncing gives some women a feeling of solidarity and sisterhood, which is invaluable and shouldn’t be dismissed, especially in light of eons of period-shaming (1).
Do women’s menstrual cycles sync?
Scientists have thoroughly studied this notion and have concluded period syncing isn’t real. We’re going to share the Cliff’s Notes version of the research.
The science on this belief dates back to a 1971 study in Nature that tracked the period start dates of 135 women in a college dorm (2). The study was considered groundbreaking at the time, but was later skewered as scientifically flawed.
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The Nature author, who was at the time a college undergrad herself, discovered that the menstrual cycles start dates became 2 days closer within a few months of living together in the dorm (2, 3). Attempting to explain the phenomena, the author suggested that airborne pheromones somehow communicated to create menstrual synchrony among the undergraduate women (2).
Since then, researchers have investigated menstrual synchrony in various groups of women, including officemates, roommates, college students, and lesbian couples — finding by and large that period syncing doesn’t happen (4, 5, 6).
Reviewing the body of research on period syncing, the author of a 1999 paper in Human Reproduction said the seeming appearance of synchrony might instead be chalked up to the natural variability in women’s menstrual cycles (4). (This is a concept that recent research underscores, which we will get to...)
The author also pointed out that in a 28-day cycle the “maximum that two women can be out of phase is 14 days. On average, the onsets will be 7 days apart. Fully half the time they should be even closer. Given that menstruation often lasts 5 days, it is not surprising that friends commonly experience overlapping menses (4).”
Here recently, the largest study of its kind on menstrual synchrony resoundingly debunked the notion, instead finding that cycles diverged with proximity (7).
Does my period change with the moon?
The belief in a connection between the lunar cycle and women’s periods is ingrained in our very language. The word “menstruation” originates from the Latin and Greek words for moon and month. However, scientific evidence on a lunar effect is mixed but overall suggests there’s probably no link.
Initially, a 1986 study of 826 women with normal menstrual cycles found that 28.3% started their periods during the new moon phase more than any other time of the month (8). Then the following year, a paper in Human Biology suggested that women tended to menstruate during a full moon, rather than a new moon (9).
In 1997, anthropological research among Dogon village women (who live without electric lighting, making them an ideal population to detect a lunar influence) found no link between the lunar cycle and menstrual cycle (10). Later, a 2013 study of more than 980 menstrual cycles also found no connection between lunar phases and menstruation (11).
More recently, data from 7.5 million menstrual cycles of women using a fertility app also found no link between moon phases and menstrual cycles (12). Instead, the study suggested that statistically speaking, assuming that periods start at random times, about half of women will have their period start within 3 days of either a full moon or new moon (12).
One reason, as we mentioned earlier, is to overcome negative experiences or stigma associated with menstruation (1). Another might be that the menstrual cycle hasn’t been the subject of serious scientific inquiry until lately.
In fact, scientists have only recently investigated the effect of external factors (stress, shift work) on menstruation or recognized that a 28-day cycle isn’t the norm (13, 14). Now, however, we know that many women’s periods change — sometimes dramatically — from month to month.
How does the timing of my period change naturally?
Researchers increasingly understand that there’s a significant degree of normal variability in the phases of the menstrual cycle. One of the most significant peer-reviewed studies on women’s periods to date gathered 1,060 cycles of menstrual data from 141 women (14).
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The study showed that close to half of women had cycles that varied by more than 7 days from month to month (14). The research also dispelled the commonly held belief that women ovulate on day 14 of their cycle, instead showing that only 25% of women ovulate between days 10 and 17 (14).
The research isn't an anomaly. Earlier this year, a massive real-world study in Nature underscored that the “widely held belief that ovulation occurs consistently on day 14 of the cycle is not correct” (15).
In spite of the research, many women feel that they really do sync with their friends, coworkers or roommates. What do you think , do you agree with the research findings?
The good news is that as science increasingly recognizes the importance of the menstrual cycle, we’re gaining more and more accurate information about this essential bodily function. And empowering ourselves with information is something we all can get behind.