Menstrual synchrony (MS) is a proposal that the duration of two or more women’s (or animals’) ovarian cycles shorten or lengthen so as to bring the timing of the onset of their menses into mutual alignment.
The effect was first described in 1971 by US psychologist Martha McClintock who hypothesized that pheromones might cause menstrual cycle synchronization. (Other mechanisms have been proposed, for example synchronization with lunar phases).
Since the publication of the original paper, however, many other academic groups have attempted to replicate the effect - some confirming its existence, and others that show it doesn't - in roughly equal numbers.
“Martha McClintock's 1971 paper, published in Nature, says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier. After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. A 2013 review concluded that menstrual synchrony likely does not exist.”