Periods sometimes overlap, but that does not establish biological synchronization. Research has not reliably demonstrated that living together or spending time together causes menstrual cycles to align.
A 2006 study analyzing 99 women living in Polish student dormitories found no menstrual synchrony and no association between social interaction and differences in menstrual onset. Cleveland Clinic likewise explains that varying cycle lengths can bring periods together temporarily before they drift apart again.
Some studies have reported synchronization, but findings have been inconsistent and methodological concerns remain. The evidence does not establish the proposed effect; this is different from proving that every possible biological influence is impossible.
Having periods at the same time is not evidence that proximity synchronized the cycles.
Anna Ziomkiewicz, Human Nature
An analysis of 99 women in 18 pairs and 21 triples found no menstrual synchrony. Social interaction was not associated with differences in cycle onset. These results weigh against the hypothesis within the studied population.
Based on: Human Nature
Cleveland Clinic
The clinical explanation distinguishes coinciding periods from a demonstrated biological effect: cycles of different and changing lengths can overlap and subsequently diverge.
Based on: Cleveland Clinic
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