Does menstrual synchrony really exist?

What is menstrual synchrony?

The belief in menstrual synchrony is based on the assumption that people who spend a lot of time together (for example, are friends, in a relationship, live together, travel together, work closely together, and so on) will start bleeding on the same day of the month.

According to this theory, it is connected with pheromones that these individuals send to each other. Some supporters of this theory even claim that alpha females and their pheromones have a decisive influence on the menstrual cycle of entire groups of people.

I would venture to say that almost every woman, or more broadly, anyone familiar with menstrual blood, has heard of synchronisation . Stories about this intriguing phenomenon have been passed down from generation to generation, girls have discussed them with bated breath in dorms and shared apartments, as well as during months-long expeditions and projects they co-created. In fact, I wondered about it myself when, in high school, I lived with two other girls and there were many situations in which all three of us got our periods at the same time.

Although the existence of synchrony had been suspected for years, the scientific community did not become seriously interested in the subject until the early 1970s.

The McClintock Effect and the Menstrual Cycle

It all started with… a mouse.

It turns out that if we place a group of female mice in a common cage, after a short time we can observe that their regularly repeated estrous cycles every 4 to 5 days will become less frequent, or may even disappear altogether. Then, if we introduce a male into such a cage or at least spread his scent (preferably the one coming from the glands of the foreskin), within 3 days most females should come into estrus.

This inspired psychologist Martha McClintock of the University of Chicago to conduct a study of 135 college women living together in a dormitory. During a detailed analysis of eight menstrual cycles of each of them, the researcher noticed that among close friends and roommates, the time between the onset of bleeding decreased from 8 to 9 days to 5 days. In randomly selected people, the difference was about 10 days.

From this point on, menstrual synchrony is also known as the "McClintock effect."

It is worth noting that McClintock did not take into account other factors, such as the date of ovulation of the studied girls, past illnesses, or stressful situations.

According to the researcher's assumptions, published in 1998, the synchronization of periods may be related to the fact that women – like other female animals – secrete special pheromones, i.e. chemical substances intended to facilitate communication between them, along with their sweat .

The vomeronasal organ , located on both sides of the nasal septum, is responsible for receiving pheromones . It has a direct connection with the brain structures responsible for emotions and instinctive behaviors, but is not connected to the cerebral cortex, which means that we react to the signals sent by pheromones subconsciously .

This would suggest that when women spend a lot of time together, they unconsciously send and receive signals to each other that encourage them to synchronize their cycles.

From an evolutionary point of view, the purpose of such a mechanism would be to ensure that women from the same tribe would become pregnant at the same time and give birth at a similar time. Then, if complications occurred during one of the births or one of the mothers had problems with lactation, the other would be able to ensure the survival of both newborns.

Perhaps it would even seem logical. However, critics of this theory point out that menstrual synchronisation also means synchronisation of fertile days , which could lead to an awkward situation in which several women ovulate at once, while only one fertile man is available.

Here, in turn, one can base one's opposition on the fact that for thousands of years people have opted for monogamous relationships, which means that one specific partner would be assigned to one fertile woman. However, the force of this counterargument weakens when one recalls the hypothesis that polygyny (the union of one man with several women at the same time) was the most common form of relationship in prehistory, and monogamy only gained popularity in democratic and industrialized societies, where social stability becomes important.

And so on, and so forth…

McClinton's research has been the cause of much heated discussion. At the same time, when a survey was conducted in 1999, as many as 80% of women who took part in it confirmed that they believed in menstrual synchrony , and 70% of them believed that synchrony strengthens female solidarity .

Period Synchronization: What Does the Research Show?

Since McClintock published her study in the journal Nature, there have been numerous attempts to test the extent to which her theory is true.

One experiment was conducted on women from the African Dogon tribe. They were observed for 763 days (according to tradition, they moved to another hut when they bled). No synchrony was demonstrated.

A similar result was obtained by researchers tracking the menstrual cycles of people in a relationship. The experiment involved 26 couples. The time interval between the first days of bleeding varied by 10 days, which is… the same amount that McClintock attributed to women she didn’t know.

You would have hoped that with the invention of the period tracking app, some kind of breakthrough would happen. After all, it meant having access to so much information at once!

Oxford University and Clue have undertaken another study. After analyzing data from over 1,500 people, they found that it is unlikely that intimate people can disrupt each other’s menstrual cycles.

In turn, the results of the 2017 Trusted Source study indicate that such synchronization is possible. What's more, it was noted that menstrual symptoms (for example, menstrual migraine) were more common in people living together. This would indicate that menstruating people can influence not only the date of menstruation, but also its course. However, it should be remembered that there was much less data for analysis here than in the previously described study.

Why is menstrual synchrony so difficult to prove?

As you can see, the occurrence of the phenomenon of period synchronization is very difficult to scientifically prove. This is due to several reasons.

First of all, we are not sure whether pheromones, on which the entire theory is based, can actually influence the date of onset of bleeding.

Secondly: the duration of the menstrual cycle is a very individual matter. We usually assume that we are talking about 28 days (similarly to the moon phase cycle), which we start counting from the moment menstrual blood appears. It is assumed that bleeding (menstruation) occurs for 5 to 7 days. But we know very well that theory is one thing, and life is another. ;)

The average cycle length is 21 to 35 days, but it can be disrupted by a number of factors, including stress , diet , weight changes , travel , and illness . Additionally, some people bleed for 3 days, while others bleed for the full 7 days.

Third, menstrual synchrony may be primarily due to… the law of probability . It’s pure math – if you have your period for one week of the month and you live with three other menstruating people, there’s a chance that at least two of them will have their periods at the same time.

All of these factors complicate research into period synchronization. So why do so many people believe in this phenomenon?

Psychology explains this with the so-called confirmation bias . Our minds like to be right, so once they believe in a hypothesis, it will be much easier for them to only absorb facts that fit it (especially when emotions are involved).

So if someone close to you has convinced you that your periods are synchronized (which could have happened completely by accident), there is a high probability that you will only pay attention to those situations that confirm this, while pushing out of your memory all the times when your periods were weeks apart.

Summary

The belief in synchronized menstrual cycles was born from the stories of many people and was later supported by the research of psychologist McClintock. The results of this experiment have never been sufficiently proven.

There is much evidence to suggest that menstruation is an individual matter, but it is not certain that menstrual synchronization is impossible. This means that we will likely have many years of research ahead of us to prove or disprove McClintock's thesis once and for all.

Until we have solid evidence for it, I personally suggest treating this phenomenon as a fascinating, yet unconfirmed curiosity about our physiognomy, based solely on the experiences and beliefs of quite a large group of people. ;)

And one more thing: yes, there is something nice about experiencing your period at the same time as someone you are close to. It is like another thread that connects you. However, if your period does not occur at the same time as someone you are intimate with, do not jump to the conclusion that there is something wrong with you or your relationship.

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