Have you ever felt that you and your besties menstruate almost at the same time of the month? The ones with a curious mind (like me) would think and ask, why do periods sync up? ‘Menstrual synchrony’ or ‘McClintock effect’, are the terms used to define a certain condition when different women who live together or who spend a big part of their days together, experience the onsets of their menstrual cycle in the same days of the month.

The Phenomenon Itself Is Very Interesting

Many women have been reported to experience the same phenomenon. This is a common observation among close friends, sisters or even among the closest co-workers. The viewpoints about menstrual synchrony are still divided. Some psychologists and anthropologists are still in doubt about how this can cause synchrony.

In 1971, Martha McClintock, a famous psychologist from Harvard University, studied the matter in detail and proved it not to be some anecdotal happening. After observing about 135 females living together in a college dorm, she came to a very significant conclusion. Her studies suggested that the girls who started to live closer as roommates or just as close friends, had got their periods started at the difference of about 5 days (earlier this difference was about 8 to 9 days).

Why Do Periods Sync up?

Why periods sync up? None could have answered it exactly. The research is still on. There have been findings; some in favor and some against this concept. Some possible reasons are briefly elaborated here.

Similar Diet

It’s quite possible that living at the same place, following same lifestyle, facing a similar stress level and particularly, following the same diet plan, are causing your periods to sync up.



Pheromones are chemical signals, released by skin glands (in under arm). Each female releases this chemical as she menstruates. These are also known as airborne chemical signals. Some studies suggest that these pheromones have the power to affect the physiology of another human being, particularly, the one who is closer to us.


Lunar Phase

There is another theory that defines moon cycle or a lunar phase to be the reason of synchrony in the menstrual cycle. The length of a woman’s menstrual cycle is quite synced with a lunar cycle (both about 28 days or so). All the females who are synced up to moon’s cycle are going to menstruate on the same time i.e. with the new moon.

Some Studies that Didn’t Find any Sync!

Some other noticeable studies and research work was undertaken to get the answer to why do periods sync up? Here are a couple of them.

  • In a research, many lesbian couples were observed closely to find the answer. But most surprising, these same-sex couples who were living together, physically and emotionally, did not show any sign of synced up periods.

  • The latest (in 2006) as well as the longest research on the same topic also resulted in against the theory presented by Martha McClintock in the ‘Nature’. Most interestingly, 186 Chinese female students lived in dorms and the research process continued for one year. Yet no menstrual synchrony was detected.

The book of this mystery is still not closed. There are many people who are working on it to get to some acceptable conclusion. Nothing is clear at the moment. While some are opposing it, there are many who believe this concept. And they even have to face a backlash for asking why do periods sync up.


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