September 8th, 2011 by Elizabeth Kissling
Guest Post by Harriet Hall, M.D.
When women live together, do their menstrual cycles tend to synchronize? It’s been a long time since I first heard that claim. I didn’t believe it, for a number of reasons. I had never observed it myself, I saw no plausible mechanism to explain how it could happen, I thought the statistics to prove it would be problematic and complicated, and I suspected that confirmation bias and selective memory might have persuaded people that a spurious correlation existed. How often do women say “Oh, look! We’re having our periods at the same time”? How often do they say “Oh, look! We’re having our periods at different times”? Now that many years have passed since my first encounter, I thought it would be fun to revisit the claim and see whether science has supported it or rejected it.
A perusal of PubMed and other Internet sources left me confused and amused.
Synchrony Is Difficult to Define
Consider that the normal menstrual cycle can vary from 21 to 35 days and can last 2 to 7 days. Consider that some women are regular and consistent, while others have variable patterns, even “regularly irregular” patterns. Consider that anovulatory cycles and other conditions often lead to menstrual irregularities that fall outside the normal range. Consider that strenuous exercise and other life events can affect menstruation. Put all that together, and you can see that often cycles will overlap simply by chance, and that it is difficult to define synchrony.
If two women have regular 28 day cycles and 7 day periods, the maximum number of days they could not overlap is 14. On average, their periods will be 7 days apart, and half the time they will be closer.
How could a 21 day cycle ever “synchronize” with a 35 day cycle? For example if you compare a woman with a regular 35 day cycle who starts on January 1 to a woman with a 21 day cycle who starts two weeks later on January 15, their next periods will coincide almost perfectly (Feb 4-10 and Feb 5-11) but they will diverge after that. Would it count if the last day of one woman’s period overlapped with the first day of another woman’s? What if half the periods coincide and half don’t? The whole thing is problematic.