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    Menstrual Synchrony: Do Girls’ Periods Really Sync?

    Updated 24 September 2019 |
    Published 16 September 2019
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    Reviewed by Anna Klepchukova, Flo chief medical officer, UK
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    Is your period tracker predicting you're due on at almost exactly the same time as your best friend? It's a long-held theory that women's periods sync, so do they? And if so, why? Various studies, some dating back to the 1970s, attribute the phenomenon to pheromones that influence the menstrual cycles of women or girls in close proximity to one another. But what does current research say? Let’s find out.

    What is considered menstrual synchrony?

    Menstrual synchrony, also known as the McClintock effect, is a process in which women who live together or in close proximity involuntarily synchronize their menstrual cycle with each other. The latest data suggests original studies validating this effect were incorrect and could not be replicated in larger populations. 

    However, many women still believe that their cycles have been influenced by a life change that brings them into contact with a new group of women. This can be daunting if you're already stressed out, suffering from PMS or period pain, and are suddenly unsure of when your cycle may begin.

    What is Martha McClintock’s effect?

    The phenomenon of women's synchronized periods has been popularly referred to as the McClintock effect after Martha K. McClintock, the researcher who published an article called “Menstrual Synchrony and Suppression” in Nature magazine in 1971. The study concluded that women living together or who work closely together synchronize their periods.

    McClintock commented that human females experienced the same phenomenon in dormitory life. The scientists challenged her to address the issue scientifically. McClintock took on the topic as her senior thesis at Wellesley and published her results while pursuing a Harvard graduate degree.

    While McClintock's papers didn't prove a causal basis for the pheromones attributed with the effect, it did offer intriguing evidence. Further research by McClintock and others (Russell et al., 1980; Stern and McClintock, 1998; etc.) pointed to pheromones causing the menstrual synchrony.

    Interestingly, other studies show that male pheromones affect women’s menstrual cycles.

    Why do women's periods sync? In a phenomenon also called the dormitory effect, menstrual cycles change for women living together in dormitories, prisons, convents, and other communities. This has been purported to impact the start date and length of menses.

    The same phenomenon, called the Whitten effect, has been noted in mice and guinea pigs. However, the Whitten effect is caused by male pheromones, whereas the McClintock effect involves only female pheromones.

    The pheromone chemicals believed responsible are released by skin glands concentrated in the armpit. These airborne chemicals don't give off an odor but are sensed by the nose's vomeronasal organ, or Jacobson’s organ. The theory goes that women release pheromones, and these pheromones signal the hypothalamus in the brain, triggering changes in other women's menstrual cycles and leading to girls and women experiencing menstrual cycles that are closely in sync.

    Status of the theory

    Although nearly 50 years of intensive investigation have passed since McClintock first published results on menstrual synchrony, there is still no conclusive evidence for the existence of this phenomenon.

    Modern studies on menstrual synchrony

    A 1992 study corrected statistical errors in McClintock's original study and follow-up studies. 

    The study found that all the previous studies had several errors that led to biased results — the researcher concluded these biases tainted the findings, leading to higher-than-expected frequencies of menstrual synchrony. The later study concluded that there wasn't any significant menstrual synchrony impacting the onset of menses or the length of the period.

    A study by Morofushi and colleagues in 2000 enlisted 64 Japanese women who lived in a dormitory. Twenty-four of the young women synchronized with their roommates in three months. Researchers concluded that the change in menstrual timing was triggered by the ability to sm