Is ‘period syncing’ real? Two reproductive health experts explain

Have you ever heard two or more women say they’re on the same cycle?

This is a common claim among women who live together, for example in a family or as housemates.

This idea that people menstruate, or have their period, at the same time is known as “menstrual synchrony”. If their menstrual periods happen to regularly align, they might describe themselves as being “in sync”.

But is menstrual synchrony possible, according to science? Let’s unpack the evidence.

The ‘menstrual synchrony’ myth

The term “menstrual synchrony” is difficult to define.

In popular culture, it’s generally thought to be the result of various unknown factors which cause two or more people to have their period at the same time. So it is supposedly due to biology, not coincidence.

Scientists also struggle to define menstrual synchrony. According to one 2023 study, it is when people’s menstrual periods start at roughly the same time, not necessarily on the same day.

But as we’ll see, research suggests being on the exact same menstrual cycle as someone else is scientifically very unlikely.

Where did this idea come from?

A psychologist named Martha McClintock likely popularised the concept of menstrual synchrony. In a 1971 study published in the journal Nature, McClintock studied 135 women aged between 17 and 22 who all lived together in a college dormitory.

Her main finding was the menstrual cycles of women who shared a room or spent lots of time together aligned over time. But this was not the case among those who lived in the same building or spent more time with men, both of which are factors that influence mating behaviour in animals.

Despite being published in a reputable and widely-read journal, today there are as many studies refuting McClintock’s 1971 study as there are supporting it. Critics mainly point to the flawed assumptions and calculations McClintock made as part of the study.

For example, when the boarders first moved in, McClintock recorded the date when each person’s period started. Several months later, she again noted the boarders’ menstrual start date. However, she did not record the length of each person’s cycle over the course of the study. That makes it hard to know whether the boarders’ periods synced purely by chance.

McClintock’s study also assumed each boarder had a standard 28-day menstrual cycle. Before the 2000s, this was widely accepted as scientific fact. But multiple landmark studies which used apps to track pregnancy and contraception show the length of a menstrual cycle can vary. We now know it commonly lasts between 28 and 35 days.

One 2017 study examined the menstrual cycles of pairs of close friends or housemates. It found three-quarters of the pairs saw the timing of their periods become less, not more, aligned. But this study was not peer-reviewed, so we must interpret it with caution.

So, why is this myth still around?

Here are three reasons.

It makes some evolutionary sense

In one 2008 study, researchers suggested menstrual synchrony could lead to greater genetic diversity among groups of primates. They argued that if multiple females are capable of reproducing at the same time, it’s less likely that one alpha male will father all offspring. In theory, this would increase the group’s long-term survival through natural selection. This is the idea that beneficial genetic mutations are passed onto the next generation through reproduction.

It’s a common misconception

Many people believe menstrual synchrony is real. This could be because they’ve noticed their period starting around the same time as a friend, housemate or family member. But they may hold onto this myth because of confirmation bias. This is the idea that people look for evidence that affirms their existing beliefs, even if they don’t do it deliberately. So confirmation bias means we’re less likely to notice the times our periods are not in sync, or to simply dismiss that possibility.

It may help women connect

One American study found 90% of women surveyed believed in menstrual synchrony. Many described it as a “magical” concept which made them feel more connected to other women. Some also said it helped them cope with the challenges of menstruation. Another study found 70% of participants said they had experienced period syncing firsthand, with most viewing menstrual synchrony as a real and positive experience.

So the evidence suggests period syncing is not scientifically supported. But it still persists in popular culture. And for some women, it may make menstruation that bit more tolerable.