There is no evidence that women synchronise their menstrual period

The study on which the claim is based had an incorrect methodology and the effect could be due to chance

Most of us have heard at some point or another that women who spend a lot of time together end up synchronising their periods. The reality, however, is that there is no scientific evidence to prove this phenomenon.

US psychologist Martha K. McClintock was the first researcher to conduct a study about women’s menstrual synchrony. It was the year 1971 and her work was published in Nature magazine.

The researcher talked about the analysis of 135 female students between 17 and 22 years old who lived together in a college dormitory. She concluded that the women who spent the most time together ended up synchronising their menstrual cycles over the course of the academic year. The article did not find any explanation for this fact and left a number of hypotheses open. But from then on, the synchronisation of menstrual periods became commonly known as the McClintock effect.

Years after the first article was published, various studies – including this one from 1991, this review from 1992 or this one from 2002 – attempted to replicate McClintock’s results, but without success. They pointed out that the original study had several methodological and design flaws, such as the exclusion of participants for no apparent reason, which artificially increased the likelihood of finding menstrual synchrony in the sample.

McClintock’s study, refuted

In the year 2006, other studies based on Polish and Chinese students who lived in different student residences did not find any synchronisation impact on their periods, claiming that women did not synchronise their menstrual cycles and that this perception could be explained by various points of convergence and divergence that occur over the course of the cycles.

“Based on the body of scientific evidence that exists to date, we cannot say that there is menstrual syncrony among women who are in close contact with one another”, Ana Robles, an OB-GYN at the Hospital del Mar in Barcelona, explains to Verificat. “It would be necessary to design new studies to evaluate this phenomenon”.

Currently, therefore, there is no scientific evidence proving that women synchronise their menstrual periods after living in the same space during a given time, according to the conclusions of various studies such as this one published in Human Reproduction, or this systematic review from The Journal of Sex Research.

The pheromone hypothesis

Various organisms, among them mammals and insects, release into the environment chemical substances capable of influencing or modifying the behaviour of other members of the same species, and this is what we refer to as pheromones.

Although McClintock hypothesised in her first study that pheromones could explain the supposed synchronisation of female menstrual cycles, “there is no evidence to support any of these phenomena [in humans]”, says Robles.

In fact, a review published in Human Nature magazine in 2006, which analysed eight studies that evaluated the possible effect of pheromones over the control of the menstrual period, concluded that there were serious doubts about the existence of pheromones that can modulate menstrual periods in humans. It also found various methodological errors in the analysed studies, such as inconsistencies in the data or statistical errors.

A review published in 2020 stated that, although pheromone communication in humans “could have a role in different systemic functions”, it has “not been definitively demonstrated”.

“The influence of ovulation depends more on the internal clock of each women – hormonal and nerve signals – rather than external hormonal factors such as pheromones”, Sergio Martínez, territorial clinical director of gynecology and obstetrics of the northern metropolitan area of the Catalan Institute of Health, explains to Verificat. In fact, for the time being scientists have “not been able to identify pheromones in humans or the receptors that would be necessary for them to modulate the menstrual cycle”, Robles concludes.