Remember when the word around the cafeteria table was that your period stops whenever you go swimming? That's a myth that most of us probably left behind when we graduated high school, but in the realm of menstrual folklore, there's one other conviction that proved a little more persistent. The idea that women who spend a lot of time together might naturally sync up their cycles has been floating around forever, and I'd be willing to wager that more women have accepted this theory—which goes by the official name of menstrual synchrony—as fact.
The backstory of this would-be phenomenon can be traced back to 1971 when a scientist named Martha McClintock published a paper on her findings after researching the subject of menstrual synchrony. McClintock came to the conclusion that women who were spending significantly more time together than they had previously did have synced-up periods. However, as the years went by and other scientists delved deeper into the subject, they found conflicting information—and now, new research can definitively put this myth (yes, myth) to bed.
In the largest study of its kind, a University of Oxford researcher analyzed three consecutive cycles of 360 pairs of women who lived together. Believe it or not, the scientist found that the vast majority—273 out of the 360—actually had a greater difference in the timing of their cycles than at the beginning. This is even further complicated by the fact that every woman's period varies in length.
So while it's a nice idea that women are naturally connected in this way (solidarity!), we can officially dub this one false. But we all still have to deal with this godforsaken fact of life every month, so there's that.
McClintock MK. Menstrual synchorony and suppression [published correction appears in Nature. 1971 Feb 26;229(5287):643]. Nature. 1971;229(5282):244-245. doi:10.1038/229244a0