Any woman can relate, and any man within eyeshot has probably witnessed the following scenario: A woman suffering from mind-numbing cramps, hobbling over to the closest female coworker’s desk for pharmaceutical relief, and bonding over their synced cycles. If women spend enough time around female friends or coworkers, they’re bound to get on the same schedule, right? Greatist examines the science behind the sync.

Sync Up or Sex Up—The Need-to-Know

Do Women's Periods Really Sync Up?

In 1971 a study speculated that regular contact with other women could affect menstrual cycles. Researchers found social interaction was key to matched cycles, and close friends and women who were in contact for a substantial portion of every day (say, hanging out or working together) were more likely to sync up than women who merely lived in proximity to each other (e.g., on the same floor of a dorm).The impact of social interaction factors on menstrual synchrony in the workplace. Weller A, Weller L. Psychoneuroendocrinology, 1995, Feb.;20(1):0306-4530. Menstrual synchrony: only in roommates who are close friends? Weller L, Weller A, Avinir O. Physiology & behavior, 1996, Mar.;58(5):0031-9384. Follow-up studies suggested pheromones were responsible and that “menstrual synchrony” (the technical term for syncing) might be just the tip of the iceberg: It’s possible periods and ovulation could be socially regulated throughout a woman’s lifespan, from puberty to pregnancy and everywhere in between.Whither menstrual synchrony? McClintock MK. Annual review of sex research, 1999, Jun.;9():1053-2528. Positive relationship between menstrual synchrony and ability to smell 5alpha-androst-16-en-3alpha-ol. Morofushi M, Shinohara K, Funabashi T. Chemical senses, 2000, Sep.;25(4):0379-864X.

But while many researchers uphold the syncing theory, there's also backlash. The pro-syncing crowd has been criticized for poor research methods, and the existence of natural synchrony hasn’t been confirmed in nonhuman primate studies.Do non-human primates synchronise their menstrual cycles? A test in mandrills. Setchell JM, Kendal J, Tyniec P. Psychoneuroendocrinology, 2010, Jul.;36(1):1873-3360.Women do not synchronize their menstrual cycles. Yang Z, Schank JC. Human nature (Hawthorne, N.Y.), 2015, Jul.;17(4):1045-6767. A critical review of menstrual synchrony research. Wilson HC. Psychoneuroendocrinology, 1993, Mar.;17(6):0306-4530. Menstrual-cycle variability and measurement: further cause for doubt. Schank JC. Psychoneuroendocrinology, 2000, Nov.;25(8):0306-4530. Is 3alpha-androstenol pheromone related to menstrual synchrony? Jahanfar S, Awang CH, Rahman RA. The journal of family planning and reproductive health care / Faculty of Family Planning & Reproductive Health Care, Royal College of Obstetricians & Gynaecologists, 2007, May.;33(2):1471-1893. Proximity and estrous synchrony in Mahale chimpanzees. Matsumoto-Oda A, Kasuya E. American journal of primatology, 2005, Aug.;66(2):0275-2565. Measurement and cycle variability: reexamining the case for ovarian-cycle synchrony in primates. Schank JC. Behavioural processes, 2001, undefined.;56(3):1872-8308.

In fact, synced cycles might not make sense evolutionarily. Syncing up in a hunter-gatherer tribe, for example, would mean that for a week or so, no woman would be able to get pregnant (not ideal when you’re responsible for procreating the human race).You mate, I mate: macaque females synchronize sex not cycles. Fürtbauer I, Mundry R, Heistermann M. PloS one, 2011, Oct.;6(10):1932-6203. Instead of having matched cycles, women might unknowingly alternate phases of “sexual receptivity” (and phases of "definitely not tonight")—meaning one woman is ready to get pregnant while another is just starting to menstruate, giving fertile women a more diverse choice of mates.Menstrual-cycle synchrony: problems and new directions for research. Schank JC. Journal of comparative psychology (Washington, D.C. : 1983), 2001, Sep.;115(1):0735-7036. (Of course, these are all biological arguments, operating on the presumption that every woman’s goal in life is to get pregnant.)

Menstrual Mystery—The Answer/Debate

Unfortunately, there isn’t a definitive answer to the science of syncing, says Frederick Naftolin, professor of obstetrics and gynecology and director of reproductive biology research at NYU. Synchronization is possible—in nonhuman animals, anyway. Manually matching up cycles (typically with the hormone progesterone) is key to many agricultural areas that rely on artificial insemination of cows, goats, and buffalos, as synchrony allows for simultaneous impregnation of a herd.Advances in modeling of the bovine estrous cycle: synchronization with PGF2α. Stötzel C, Plöntzke J, Heuwieser W. Theriogenology, 2013, Feb.;78(7):1879-3231. But evidence for naturally occurring human synchrony hasn’t been confirmed via adequately controlled, randomized trials.

Still, some researchers strongly maintain menstrual synchrony is real.Menstrual synchrony can be assessed, inherent cycle variability notwithstanding: commentary on Schank (2001). Weller A, Weller L. Journal of comparative psychology (Washington, D.C. : 1983), 2003, Feb.;116(3):0735-7036. Menstrual synchrony and cycle variability: a reply to Schank (2000). Weller L, Weller A. Psychoneuroendocrinology, 2002, Jun.;27(4):0306-4530. Anecdotal evidence certainly seems to support the idea of syncing, says Martha Thomas, associate residency program director in the OB/GYN department at York Hospital/Wellspan Health. And it’s hard to say how factors like stress, sexual partners, and birth control play into the syncing game. If synchrony does exist, it’s possible these factors override it, making matched cycles appear less common than they might actually be.The effect of job stress and job interdependency on menstrual cycle length, regularity and synchrony. Matteo S. Psychoneuroendocrinology, 1988, Apr.;12(6):0306-4530.

But this is mostly speculation, stresses Thomas. Most contemporary research holds that it’s unlikely women will actually sync up month after month after month, especially because the length of individual cycles can vary so dramatically.Avoiding synchrony as a strategy of female mate choice. Schank JC. Nonlinear dynamics, psychology, and life sciences, 2004, Jun.;8(2):1090-0578. What seems most likely is that women don’t actually synchronize but rather have the occasional synced period when their cycle lengths randomly overlap. Surprised? Us too. But we’ll still share the Midol.

Thanks to Dr. Frederick Naftolin and Dr. L. Martha Ann Thomas for their help with this article.

Originally published September 2012. Updated February 2016.

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