Talk to a woman about her period and she will probably give you an example of the time her cycle aligned with a friend, partner, colleague or family member. Many of us have noticed that the closer we get to another woman the more we seem to get crampy, grumpy, tired, bloated and spotty at the same time. It’s as if our uteruses, in a monthly show of solidarity, are saying hey, why not go with the flow? Let’s do this painful, stigmatised, and bloody expensive thing together, and take advantage of the three-for-two tampon offers in the process.

However, a new scientific study – thought to be the largest of its kind – has found data showing women’s periods do not synchronise when they live together after all. The study – carried out by period tracking and fertility app Clue in partnership with the University of Oxford – received 1500 responses, which were narrowed down to 360 pairs of women. Analysing three consecutive cycles in each pair, the research found the vast majority – 273 pairs – had a greater difference in period start dates at the end of the study than at the beginning. In other words, menstrual syncing is a myth up there with periods being tied to the waxing and waning of the moon. Not only that, women’s menstrual cycles are more likely to diverge than come together over time.

“It’s very unlikely that cycle syncing is a real phenomenon,” says Clue’s data scientist Marija Vlajic. “Menstrual syncing amongst the sample we had did not exist. We’ve also done some statistical tests and found that the difference in cycles actually grows. This doesn’t mean that pairs go out of sync – it means they were never in sync in the first place. It’s the nature of two mathematical series that keep repeating: the series will diverge as the numbers grow.”

This has been my experience. I have been with my female partner for 13 years, living together for 11 years. In that time, our periods have never synced for more than a month or two, because our cycles are different lengths.

“Exactly,” Vlajic says. “So there will be a time every six months, say, when your periods sync but that doesn’t mean the difference is getting smaller.” Has Vlajic ever experienced her period syncing with another woman? “My background is scientific,” she notes. “So when I say to my friends that I have my period and they have theirs too I don’t conclude that we are syncing. I just think it’s information bias; our brains looking for patterns.”

Still, the belief in menstrual synchrony persists, with a study published in 1999 revealing that 80% of women believed in the phenomenon and 70% saying they enjoyed it. The editor who commissioned this piece told me that she syncs with her sister whenever they spend time together and gets her period at the same time as her closest colleague. The idea has been around for centuries, though because menstrual health has long been overlooked by the scientific research community it was not until 1971 that it was first documented in a study.

In a Harvard research paper titled Menstrual Synchrony and Suppression, psychologist Martha McClintock tracked 135 female college students living in the same dorm and found “a significant increase in synchronisation of onset dates”. She concluded: “the evidence for synchronicity is quite strong, indicating that in humans there is some interpersonal physiological process which affects the menstrual cycle.”

The idea that pheromones enable women to become sexually receptive at the same time has been researched in various groups as well as in rats, baboons and chimpanzees. Themain evolutionary explanation is that it permits female species to avoid being monopolised by a single dominant male. But McClintock’s paper has been discredited on methodological grounds and a whole host of other studies, like this new one, continue to prove that menstrual synchrony is a myth.

So why won’t we let this one go? And how does Vlajic explain all the anecdotal evidence of our periods synchronising? “I like the idea myself of this dominant super uterus in a group of women that makes everyone adjust their cycles,” she admits. “I can see how it gives you a special connection with a woman to go through that at the same time. It feeds into a feeling of connection, support, and sisterhood. Even though we do it every month, t Periods are personal and the thought of sharing with someone makes the idea powerful. That’s why we continue to look for patterns even when they don’t exist.”

Chitra Ramaswamy