Menstrual Synchrony: Female Phenomenon or Fake Science?
Alicia Armeli


If you’ve ever had female roommates or a house full of sisters, you’re probably well aware of the bond we all share surrounding the comings and goings of Auntie Flow. A rite of passage, menstruation provides a unique sisterhood.

But could our periods be connecting us ladies on a level we may not even be aware of? Menstrual synchrony—or the theory that women who spend a lot of time together also get their periods together—has been studied for decades. And contrary to popular belief, many researchers place this so-called phenomenon on the same level as bunk science.

The idea of menstrual synchrony started back in 1971 when University of Chicago psychologist Martha McClintock observed 135 girls living in a dorm of a suburban women’s college.1 Three times throughout the academic year, McClintock interviewed the girls about their period start dates and social interactions.

Results published in Nature showed that girls who spent the most time together—either as roommates or closest friends—were more likely to start their periods around the same time. This synchronicity, labeled “the McClintock Effect,” was seen to happen more so in April as the academic year unfolded—and more time was spent together amongst girls—versus early on in October. McClintock attributed these results to pheromones or body chemicals humans unknowingly give off that influence the behavior and physiology of other humans in close proximity.

Following this study, other researchers tried to verify McClintock’s results by studying menstruation in other female groups, including women living in dorms, cohabitating lesbian couples, tribal women, as well as other mammals like chimpanzees.2

As a whole, results from these studies remained inconsistent. It seemed that for every study supporting menstrual synchrony, there was another study that refuted it. Several researchers concluded that menstrual synchrony was probably due to coincidence or “pure happening” rather than female phenomenon.2

Probably one of the most fascinating attempts to prove menstrual synchrony exists emerged in 1980. Russel, Switz, and Thompson swabbed cotton pads soaked with an alcohol-underarm perspiration mixture obtained from a female donor under the noses of participating women to see if the smell of pheromones changed the onset of menstruation.3 Results showed a significant shift in the timing of the participants’ periods—a reduction in the average start date from 9.3 days to 3.4 days—that reflected more closely the cycle of the donor.

Further work by McClintock found that women exposed to female donor underarm secretions resulted in a change in menstrual cycle length—but results have been questionably significant.4

Menstrual synchrony may exist, but it could be attributed to other factors. A study led by Jhanfar et al. found that although specific pheromones released from the underarms weren’t related to menstrual synchrony, things like pheromones located in the vagina that are given off by way of menstrual blood and vaginal discharge might play a role.5 Other studies have found that more important factors influencing menstrual cycle differences are body weight and period irregularity, not so much pheromones or social interactions between women.6

Overall, the evidence to support menstrual synchrony is weak and more research is needed to solidify its existence and which factors, if any, influence it. However, this doesn’t mean that this biological commonality can’t serve as a time for women to connect with themselves and each other.


  1. McClintock, M. (1971). Menstrual synchrony and suppression. Nature, 229: 244-245.
  2. Matei, C., Tampa, M., Sarbu, I., et al. (2015). Menstrual synchronizing: myth or reality? Gineco EU, 11(39): 31-32.
  3. Russell, M., Switz, G., & Thompson, K. (1980). Olfactory influences on the human menstrual cycle. Pharmacology, Biochemistry, and Behavior, 13(5): 737-738.
  4. Stern, K., & McClintock, M. (1998). Regulation of ovulation by human pheromones. Nature, 392:177-179. doi:10.1038/32408.
  5. Jahanfar, S., Awang, C., Rahman, R., et al. (2007). Is 3α−androstenol pheromone related to menstrual synchrony? Journal of Family Planning and Reproductive Health Care, 33(2): 116-118.
  6. Ziomkiewicz, A. (2006). Menstrual synchrony: fact or fiction? Human Nature, 17(4): 419-432. doi:10.1007/s12110-006-1004-0.