Menstrual synchrony in female couples.

The menstrual cycles of cohabitating women have been found to synchronize, possibly through social and pheromonal mechanisms. The extent of this phenomenon, menstrual synchrony, was examined in 20 couples of lesbian women. Synchrony was very frequent, with half the subjects menstruating within 2 days of their partner. Factors related to the degree of synchrony included mutual activities, friendship and menstrual regularity.