Do women’s periods sync?

Have you ever noticed your period ‘syncing up’ with other women and people with uteruses that you live with and/or are friends with? Maybe you experienced this at uni, or with a flat mate, or with someone you spent a lot of time with. 

There is a popular old wives’ tale which suggests that your menstrual cycle will sync with other women you live with or spend a lot of time with, but is there any truth to it – or is it just a myth?

Whilst anecdotal evidence may point to the fact that many menstruating people notice that they get their period around the same time as their friends, there hasn’t been much scientific data to back this up. One attempt to do this was a research paper in the 1970s.

“This (idea that your menstrual cycle will sync up with other people with uteruses who are close to you) is based on a 1971 research paper based on 135 students, and it has not yet been proven that it happens – rather it is due to the mathematical probability that with a number of women living together, eventually their period will occur around the same time. Other factors can affect when our periods occur, such as stress, weight changes, illness, and exercise for example,” says Dr Hana Patel, a GP and mental health coach.

In 1971, American psychologist Martha McClintock published a paper on menstrual synchrony among women living together in dormitories at Wellesley College. The paper, which was published in the British scientific journal Nature, stated that menstrual cycle synchronisation happens when the start of the menstrual cycle (i.e. the start of the period) of two or more women become closer together in time than they were several months earlier. 

She found ‘significant statistical synchronisation’ of menstrual cycles when studying pairs of close female friends and roommates, and that they were more likely to have their menstrual cycles synchronised than when comparing random women who hadn’t spent time with each other. There was no more or less synchronisation when comparing roommates and close friends: it seemed either relationship contributed to menstrual synchronisation.  McClintock suggested that pheromones could be responsible for menstrual synchronisation. Her study has been criticised for not adding the element of chance into her findings.

Other researchers have tried to replicate this study, with various results. One study of 20 lesbian couples found that half had synchronised menstrual cycles within two days of each other. However, another study of lesbian couples did not find any evidence of synchonrisation. In 2016, a study of 186 Chinese women living in dorms for over a year did not find that the women’s cycles synced up, and that any perceived synchronicity was due to chance.  

Then, in 2017, Oxford University researchers used an app which allowed 360 pairs of women to track their menstrual cycles. They found that 273 of the pairs in the study actually diverged rather than synchronised, and that women living together were no more likely to synchronise than other pairs. They concluded that menstrual synchronisation is a myth based on chance. 

“Your menstrual cycle is controlled by your hormones and there is currently little or no evidence to show that women can disrupt each other’s menstrual cycles by being in close proximity to one another. As with many women’s health issues, menstrual synchrony deserves more attention and research, despite how difficult it may be to prove or disprove,” says Deborah Evans, a pharmacist with over 30 years of experience.

It’s important to note that not every menstrual cycle is the same. Whilst the typical cycle lasts for 28 days, anything from 21 to 40 days is considered ‘normal’. Your cycle can also change from month to month, depending on factors like stress, sleep, diet, contraception and so on. Perhaps if you are living in the same environment as someone else those factors may be similar. 

Also, if one person has a shorter cycle and another has a longer one, they may seem to ‘sync up’ their periods, but they may actually be on a very different cycle. Some people may also ‘sync up’ for one month and then slowly fall out of sync, depending on their individual cycles.

McClintock suggested that pheromones may play a part in menstrual cycle synchronisation. It is true that human beings excrete pheromones and that, on a subconscious level, men and people with testicles may be able to ‘smell’ when someone is fertile, as that would be the prime time to try and mate with them to increase their chances of reproducing. 

During the ‘70s, there was an idea that female menstrual cycles would sync up so that one man would not be able to reproduce with all of them, because they would all be fertile at the same time. This would help them avoid becoming a harem for one male, and potentially could contribute to more genetic diversity. As of yet, there is no evidence that pheromones between people with uteruses have any effect on when they start their periods. 

Given the current evidence, if the average menstrual cycle is 28 days then it’s more likely than not that your period would arrive at the same time as those around you some of the time, and potentially if you believe in menstrual synchronisation you’re more likely to remember the times it did synchronise rather than the times it did not. This does not mean that menstrual cycles definitely don’t sync up, but until more research is done we just don’t know for sure.