Concurrent Menses with Roommates or Coworkers? Here’s Why?

If you’ve ever spent a lot of time with another woman (a roommate, a sister, a best friend, etc.), you may have noticed that your periods sometimes seemed to happen together. And you may have chalked it up to menstrual synchrony — the idea that women can get their periods at the same time if they are together often enough.

However, is menstrual synchrony a theory backed by science? Significantly, there are a number of studies over the course of almost 50 years that say maybe not.

The earliest study of menstrual synchrony took place in 1971 and was published in Nature magazine.  In that study, University of Chicago psychologist Martha McClintock studied the menstrual cycles of 135 women living together in a college dorm.¹ McClintock interviewed the girls 3 times throughout the academic year to analyzed when their periods started and how closely they interacted with each other.

McClintock’s results seemed to prove the existence of menstrual synchrony, or “the McClintock Effect,” as it was dubbed back then. She found that there was an increase in synchronized periods among girls who were friends or roommates. In addition, she found that this synchronicity was more common toward the end of the school year than toward the beginning. McClintock theorized that the synchronicity of the periods occurred because of the girls’ exposure to each others’ pheromones, which are chemicals emitted by the body.

Over the years, a number of other studies have been conducted to try to support McClintock’s findings. These studies have moved beyond McClintock’s group of college students and included all kinds of groups, from co-habitating lesbian couples to other groups of college students to chimpanzees.

Unfortunately, these studies have failed to create a consensus regarding whether or not menstrual synchronicity is real. Some studies seem to support it, while others do not.

In 1980, a group of three researchers used underarm perspiration from a woman to test whether a woman’s pheromones could affect the menstrual cycles of other women.

The researchers, Russel, Switz, and Thompson, mixed this perspiration with alcohol, soaked cotton pads in the solution, and swabbed the pads under the noses of study participants. Interestingly, it seemed as if the process had an effect: Participants’ periods started in an average of just 3.4 days as opposed to an average of 9.3 days.

Unfortunately, this theory was later called into question by similar work conducted by McClintock. Her work yielded similar results to that of the three researchers. However, her results were far less significant than theirs, leaving the question regarding the influence of pheromones still unanswered.

The second study that is particularly interesting regarding menstrual synchrony is one conducted by Jhanfar et al. This study found that it might not be perspiration pheromones influencing the timing of a woman’s menstrual cycle, but pheromones from the vagina.

In addition, it may not even be the case that pheromones have the biggest say in when a woman’s period begins. A number of other studies have been conducted that determined that factors such as body weight play a larger role than pheromones or social interactions.

As a result, of all this research, the theory of menstrual synchrony remains unproved. Some researchers now believe that menstrual synchrony is simply coincidence.