Menstruation timing: truth or myth?

Many women have ever had an experience that seems to confirm the timing of menstruation. Almost all of them have a story to tell about it, claiming that their menstrual cycle aligns or matches that of other people around them.

In this article we will know what is the synchronization of menstruation and the research that has been carried out in this regard to establish whether it is a scientifically based fact or just a myth.

What is menstruation timing?

Menstruation timing is the term used to refer to the belief that women who live together or spend a lot of time together have their period on the same days. It is also known as menstrual synchrony, period timing or McClintock effect, by the name of the first scientist to propose a theory about it

The timing of menstruation is based on a hypothesis. It is proposed that when two women interact in the same physical space and one of the two menstruates, it influences the other. This then triggers the period in the second, eventually bringing both cycles into alignment.

But is this true? Do women's periods become synchronized by living together or staying together for a long time? Is there a cause that explains it or is it just chance? Let's see if there is a scientific basis for this.

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What does science say about menstruation timing?

It has been several decades since research on menstruation timing was first presented to the scientific community. Other studies have subsequently been carried out. Anyway, there are still few investigations aimed at demonstrating whether or not there are patterns in the menstrual cycle of women living together (roommates, family members, partner).

The start date of menstruation was the key piece of information used by the first scientist to propose the theory.

McClintock effect

As noted, the pioneer on this issue was the American psychologist Martha McClintock, who proposed the idea that the timing of menstruation began to be taken into account by the scientific community. In her study, she interviewed and observed 135 women who lived together in a college dorm over the course of a year.

His conclusion seemed to confirm that the belief in the timing of menstruation had a basis, for the difference between the start dates of the period tended to decrease in those women who spent time together. McClintock even found that the beginning of the period was closer between friends and colleagues than between strangers taken at random.

From this, he proposed the hypothesis that pheromones affect each other, having a direct effect on menstruation. This triggered a debate, for and against.

Some research with humans and primates seemed to replicate the results. But there have also been other studies that have found no evidence in favor of timing.

Most criticisms of McClintock's work suggest that the notion of timing is vague. Furthermore, the author only considered the start date of the cycle, without taking into account chance or other factors (when the woman is ovulating, for example).

Other hypotheses

Other models or explanatory hypotheses have been proposed for the coincidence in menstrual periods, alternative to McClintock's theory. Among them we have the following:

  • Emotional hypothesis: women synchronize periods in an empathic way, as a defense mechanism.
  • Environmental influence: In synchronization, influence is not established from one woman to another, but from the environment. In this sense, the coinciding menstrual cycles would respond to the fact of having the same hours and activities, being under the same temperature and humidity, among other factors.
  • Lead uterus: there is also a belief that some women might have an alpha uterus that plays a leadership role in a community. On the other hand, there would be the beta uterus, which adapts or follows the rhythm.

Recent studies

McClintock has continued to conduct research to corroborate his hypothesis. In this sense, the author took odorless compounds from the armpits of women in late follicular phase and proposed that pheromones would influence the acceleration of the preovulatory peak in female recipients, who would shorten their menstrual cycles.

On the other hand, axillary compounds from the same donors, but collected in the menstrual cycle, had the opposite effect, delaying the increase in luteinizing hormone in the recipients and lengthening the menstrual cycle.

However, a scientific study was conducted on women living together in a college dormitory, who became in sync with their roommates over a period of time. The sensitivity to pheromones was compared between synchronized and non-synchronized women, but no differences were found between the two groups in the detection of pyridine and 5-alpha-androstenone.

Period tracking apps are currently being used, which store large amounts of records of women's cycles. In this sense, there is a lot of data available to verify if the timing of the period is a real fact.

An empirical analysis of that data was developed by the application Clue with the collaboration of a medical team from the University of Oxford. However, the results also do not support McClintock's initial conclusion.

Keep reading: False beliefs about menstruation

Is menstruation timing a product of chance?

A standard menstrual cycle is 28 days, with 5 to 7 days during which bleeding is experienced. But there are cycles of 35 and even 40 days. Some women have only two or three days of bleeding, on the other hand.

Therefore, it is quite likely that at some point a woman will have her period at the same time as others with whom she lives, studies or works. The timing of menstruation would be rather a subjective interpretation of the laws of probability.

And it is that until now there has been no significant evidence that implies discarding the most logical idea: the coincidences in the period do not respond more than to chance.

Cell phone apps for tracking menstrual cycles are useful tools to keep track of timing.

What can affect the menstrual cycle?

One of the most widely considered arguments in favor of menstruation synchronization is that, on certain occasions, some women vary the duration of their period, shortening or lengthening it. However, there are many factors that can affect or condition the rule to be late or early.

Among the factors that can contribute to the variation in the duration of the menstrual cycle are the following:

  • Sleep disorders.
  • Overweight or obese
  • Eating Disorders.
  • Intense physical activity: in highly competitive athletes.
  • Consumption of alcohol, drugs, drugs (aspirin, anticoagulants, anti-inflammatories).
  • Stress and anxiety.
  • Diseases and conditions: endometriosis, uterine fibroids.
  • Birth control methods: birth control pills.

The social aspect of menstruation timing

Why does the belief in menstruation timing persist? Perhaps, as with so many other things, a perception bias occurs. That is to say, we see only the times when it matches and ignore the times when it doesn't, to support beliefs.

This could be linked to the sense of tribal solidarity. In this regard, it has been hypothesized that synchronized menstrual cycles could provide women with advantages to survive in certain environments. As well there is a need for human beings to connect their physical experiences with the emotional part.

The synchronization of periods is still a social construction, a socially shared idea. However, it is positive if it helps to reinforce the feminine bond and the connection with nature itself, normalizing the perception of menstruation.