text: how come when a group of girls are all living together their period gets synchronized?   the reason behind synchronous menstruation is that women (and men) secrete odorless molecules, called pheromones, in a manner as common as sweating. in fact, the sweat of many mammals contains pheromone chemicals. the molecules from one woman bounce and jiggle their way airborne and waft to another. receptors in the nose detect the pheromones and the second woman responds by shortening or lengthening her menstrual period. gradually, over the course of a few months, the periods of all the women living together synchronize in this way. men have an effect too, men's pheromones also affect women's menstrual periods. women respond to these pheromones by shortening their periods and having them more regularly. this was noted among women who had lengthy cycles pre-marriage and once they got married their menstrual cycles became shorter and regular.