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.
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Acknowledgements: We thank Patti R. Rosenbloom for initial discussion of the research, Amir Ray for data collection, Dr. Israel Ziv for statistical advice and Prof. Abraham J. Susswein for thoughtful advice and criticism in all stages of the research. This research was supported by grants from The Israel Ministry of Absorption and The Israeli Council for Higher Education -Yigal Alon Fund to A. Weller.