Two roommates started on the same Tuesday. A whole flat, or a whole office, seemed to run on one calendar. Stories like these feel like proof, and for fifty years the idea had a scientific name to go with it. The researchers who went back and checked found that periods don’t sync. The overlap people notice comes from arithmetic, shared routines and memory.
Where the idea came from
In 1971 the psychologist Martha McClintock published a short paper in Nature called Menstrual synchrony and suppression. She had collected period dates from 135 women living in a college dormitory over a school year and reported that close friends and roommates started their periods closer together as the year went on. She suggested that something passed between people who spent time together, probably a scent signal. The effect took her name, and the story travelled a lot faster than the follow-up work.
What happened when people checked
In 1992 H. Clyde Wilson published a critical review of the synchrony studies in Psychoneuroendocrinology. He went through how the original papers had measured the gap between two people’s start dates and found that the method itself produced apparent convergence, even from random data. With the method corrected, the effect went away.
Two studies in 2006 tested the idea directly, with better methods. Zhengwei Yang and Jeffrey Schank followed 186 women living in dormitories in China for a year and published the result under the title Women do not synchronize their menstrual cycles. The amount of lining up and drifting apart they saw was what chance predicts. In the same issue of Human Nature, Anna Ziomkiewicz asked whether synchrony was fact or artifact and, with a different group of women living together, reached the same answer.
The largest set of real cycle data on the question came from a period app. In 2017 Clue worked with Dr. Alexandra Alvergne at the University of Oxford and asked users who believed they were syncing with someone to take part. Over 1,500 people answered, and 360 pairs had at least three cycles each in the same stretch of time. In 273 of those pairs the gap between start dates was bigger at the end than at the beginning. Across all 360 pairs the average gap went from 10 days to 38. Living together made no difference. The study was a self-selected pilot and never went through peer review. It still pointed the same way as everything since 1992, and the people in it had all believed they were in sync.
Why it feels real anyway
Two cycles of different lengths can’t help lining up now and then. Take a friend with a 28-day cycle and one with a 30-day cycle who happen to start on the same day. A month later they’re two days apart. Seven months later they’re two weeks apart, which is as far apart as two cycles can get. Then the gap closes again, and about 15 cycles after they first matched, they match again.
So by arithmetic alone those two friends start within a couple of days of each other for a few months every year, and since a period lasts around five days, their bleeding days overlap far more often than that. The Cleveland Clinic makes the same point. Memory does the rest. People remember the months that matched and don’t count the months that didn’t.
People who share a life do share the things that move a cycle. The NHS list of reasons a period comes late or not at all is mostly stress, weight change, exercise and illness. A hard month or a long trip can act on two people at once. That shifts both cycles in the same direction in the same month without anything passing between bodies. Hormonal birth control does the opposite. It fixes a cycle to the pack, so two people on the pill hold whatever gap they started with.
If you want to know where your friends are anyway
The wish behind the myth is a reasonable one. It helps to know that the friend you’re travelling with is due the week you land, or that the housemate who’s gone quiet is on day two. Shared dates tell you that.
We build Plum, a group period tracker, and sharing your period with friends is the feature we built it around. You create a group, send a private invite link to the people you’d tell anyway, and everyone in it sees each other’s period days and predicted days on one timeline. Members see dates, and exercise days if you track them, and nothing else. Your symptoms, moods and fertile window stay in your own account. Sharing is on from the moment you join. Every group has its own switch, and you can turn it off or leave without asking anyone.
The same rules apply as for sharing with a partner, which our guide to sharing your cycle goes through. The person whose cycle it is decides. It has to be easy to stop. And a predicted date is an estimate, so nobody gets to use it as an explanation for a mood.
Plum isn’t the only app that does this. Our comparison of period tracker apps that share with friends covers fifteen of them, including the ones built for a partner rather than a group.
