Many women who live, study or work together for a long time believe that their periods synchronize and begin to occur at the same time. We decided to check how scientifically sound this is.

Many people have no doubt that women who spend a lot of time together have their periods synchronized. When you ask about period synchronization on Google, the following options appear: synchronize periods with your sister, friend, daughter or girlfriend. Questions about this are asked on forums, for example on “Yandex.Q» or Quora, and in women's groups there are often jokes about this. Many believethat synchronization is explained by female pheromones, which interact with each other during menstruation. There is also a theory about alpha uterus: supposedly one of the women in the group has a dominant uterus, which sends a signal to others through pheromones to begin menstruation. Even about the phenomenon of menstruation synchronization exists separate article in English Wikipedia.

Although in some traditional societies the myth about the synchronization of women's periods existed, it spread in modern media only in the 1970s. This happened thanks to the research of American psychologist Martha McClinton, who studied pheromones.

In 1971, McClinton published in a reputable scientific journal Nature the results of a study that examined the menstrual cycles of 135 women living in an American college. McClinton found that girls who live together in a dorm or are close friends for several months reduce the difference between the first day of their menstrual cycle, meaning their periods are almost synchronized. The author of the study suggested that this is influenced by the time that women spend together, as it allows pheromones to interact more closely with each other.

Since then, scientists have begun to study in detail the synchronization of menstrual cycles in different groups of women: among colleagues, neighbors, in families and lesbian couples. Studies have also been conducted on other species, such as rats or chimpanzees. Theories began to emerge as to why this was happening. According to the popular Of these, synchronization of menstruation could have appeared during evolution: supposedly, if women’s menstrual cycles begin at the same time, then they also become sexually available at the same time. And this, theorists believed, leads to the fact that for women the risk of ending up in the monopoly of one man is minimized. If everyone has their periods at the same time, then everyone will have the same time for fertilization. This means that a man will not be able to have sex with everyone at once and women will be able to have sexual relations with different partners, thereby maintaining genetic diversity.

In the 1970s, in the wake of rising feminism, such a theory looked attractive. However, later researchers began to doubt not only the theory explaining the phenomenon, but also the phenomenon itself. First, in McClinton's methodology noticed several serious mistakes: for example, she conducted a study on women in whom the initial difference between the first days of the cycle was given and the same. In addition, McClinton did not take into account respondents with irregular menstrual cycles, although for a significant part of the population this may be the norm. (For example, in South Korea such women in 2019 it turned out more than 14%.)

But most importantly: the author did not check and did not take into account the fact that synchronization of menstruation can occur due to chance.

Given the methodological errors, the new studies did not produce the same results as McClinton's. In 2006, a large study with 186 women was done in China. Its authors discovered, that most of the synchronizations could occur simply due to the non-zero probability of coincidence of random events, and could not confirm the phenomenon.

In 2017, an even larger study held at the University of Oxford using the Clue mobile app to help women track their menstrual cycles, ovulation and plan for pregnancy. The study looked at about 1,500 respondents, all of them were divided into 360 couples who knew each other well. Scientists have not identified any synchronization. Although this work has not passed the blind peer review stage, much of the scientific community is now believesthat the phenomenon of synchronization of menstrual cycles requires serious scientific substantiation and is too well explained by simple chance.

The fact is that women have menstrual cycles of different lengths, the same applies to the period itself. Because of this, at some point girls find that their periods are closer together, but then their periods become out of sync again (and come back together later). By noting only periods of coincidence, women think that their periods coincide due to the fact that they spend a lot of time together.

This is what a random coincidence of women's periods looks like.