Menstrual Synchrony

Menstrual synchrony is a phenomenon wherein the menstrual cycles of women who live together reportedly becomes synchronized over time. The existence of menstrual synchrony has not been definitively established, and studies investigating it have been controversial.

Diagram of the Menstrual Cycle

Scientists at the Sonoma State Hospital Brain Behavior Research Center in California identified several women who were believed to be menstrual pacesetters – they made other women conform to their cycles. The scientists placed cotton pads under the dominant women’s arms for a day, and then wiped the pads on the upper lips of five female subjects three times a week. Within five months, four of the recipients were menstruating at the same time as their donors.

However, the interaction of theorized menstrual synchrony with differing cycle lengths has not been explained. Two women with cycle lengths that differed by two days might initially begin menstruating on the same day, but the next month would be two days apart, the month after that four days, and so on. This leaves us with the question how variable this so-called synchrony can be.

Why synchrony – if it indeed does – occurs is pretty much a total mystery. The only published common theory treats it as an evolutionary holdover from prehistoric times, when it was common for men to take multiple mates and efficient reproduction was essential to the survival of the species.

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