The belief that the women who spend a lot of time together sync their period has been held even for centuries. Even now, many people mention it casually. A study that was published in 1999 found that 80% of women believed in period-syncing and 70% enjoyed the phenomenon. Though that survey was conducted 20 years ago, the theory is still widely circulated today. However, women’s health has been a neglected area of study so it’s only being researched recently.
The period-tracking app Clue received 1500 responses for its call for participants who thought that their menstrual cycles were syncing with someone else. They narrowed the sample down to 360 pairs for a variety of reasons, such as needing data for at least three consecutive period cycles and not being on birth control. Each woman had some sort of close relationship with the other member of the pair. They could have been friends, siblings, roommates, partners, mother-daughter pairs or colleagues. About one-third of the participant pairs lived together.
The majority, 273 pairs, had a greater difference in the start of their periods than at the beginning of the study. Only 79 pairs actually had their period start dates come closer together. Living together didn’t make a difference. 37% of the pairs with diverging cycle dates lived together, while 24% of the pairs with converging dates lived together. In statistical tests, the analysts found that that the difference in cycles actually increases. Their period’s don’t go out of sync because they never were in sync in the first place. If you have two random repeated series, the series diverge as the numbers become larger. Many women have differently lengthed cycles, not all are exactly 28 days long. Since an average period is five days long, the period can often match up just by chance.
There are a few other reasons why women might think that their periods are syncing up. Firstly, they may notice it more and attach more significance to cases of period-matching as opposed to non-matching. It also might be a way to feel solidarity and support through the uncomfortable period that almost all women go through.