Probably not: when scientists followed women sharing dorms, their cycles didn't adjust to each other. Overlaps still happen all the time, and simple arithmetic explains them. If you and your friend keep lining up and then drifting a couple of weeks apart, that's normal.
Where did the idea come from?
In 1971 the researcher Martha McClintock wrote in the journal Nature about 135 students living in one dorm. By her count, friends' period start dates moved about 2 days closer over 4–6 months. The story was so charming that people still tell it.
What did people find when they checked?
Later, people found mistakes in the maths: it didn't properly account for overlaps that happen by chance. When McClintock's data were reanalysed, the dates got no closer than chance would predict.
Newer studies didn't find syncing either. That includes 186 students in China followed for over a year and roommates in Polish dorms. And in data from the Clue app, 273 of 360 pairs (friends, sisters, mothers and daughters, partners, roommates, colleagues) ended up further apart than they started.
Why does it feel like more than chance?
A month when you match sticks in your memory: "no way, together again!" Nobody celebrates the months when you were two weeks apart. That's how memory gathers evidence for a nice story.
What about smell and pheromones?
One idea was that we pick up on each other through smell. There's still no solid evidence that a friend's scent can shift your cycle.
Common questions
What if we've matched for six months?
That happens when your cycles are almost the same length. If they differ by one day, say, your dates slip apart by a day a month and stay close for a long time, until the difference adds up.
What about the moon? Maybe we both follow it?
There's one moon for everyone and everyone's cycle is different: if the moon were in charge, we'd all start on the same day. Among Dogon women in Mali, who had no electric light and slept outdoors, researchers found no link between periods and the moon's phases.
But I read a study that did find syncing
There were studies like that, and people argued about them for years. But studies that properly allowed for chance overlaps found no effect, not even a weak one.
In short
- Log your periods in HeySis and go by your own calendar
- Matched with a friend? Share the hot water bottle
- Cycle shorter than 21 or longer than 35 days? See a doctor
Source: Human Nature: Women do not synchronize their menstrual cycles
Educational content; it does not replace professional medical advice.