Why sisters and friends share simultaneous menstrual cycles


by Megan Stoddard
October 1, 2010

If women spend a lot of time together, are emotionally close, or both, they are likely to find themselves sharing simultaneous menstrual cycles. If they did not have simultaneous menstrual cycles in the beginning, they will after a few months. Each woman’s cycle will move by a few days until they are getting their periods within, at most, three or four days of each other.

This phenomenon, called menstrual synchrony or synchronicity, may happen to sisters, mothers and daughters, roommates, or close friends. It may happen to a pair of women, or it may happen to several who are all close to each other. It is such an intriguing concept that menstrual synchrony boasts its own Facebook page, with (as of October 1st, 2010) 89 followers.

Menstrual synchrony was identified in 1970 (findings published in 1971) by researcher Martha McClintock, based on interviews with students at Wellesley College, a women only college. Over the course of an academic year, McClintock surveyed 135 women, ages 17 to 22, who lived in the same residence hall. Her subjects reported when each of their periods between late September and early April began and how long it lasted. Analyzing the data, McClintock found that the women’s periods tended to come at about the same time as those of their roommates and of women they identified as close friends.

This was not necessarily so at the beginning of the school year. Over several months, many of the women had their cycles adjust, so that they began to menstruate in sync with their roommates and friends. Those who were on birth control pills did not experience any changes. McClintock initially included them in the study because, although women on the pill consistently maintain the same menstrual cycle and would not have it subject to change, she was uncertain how they might affect the cycles of others. At the end, she concluded that women on birth control pills did not affect others’ menstrual cycles, as well as not being affected themselves.

Later studies, including a follow up study by McClintock, have offered a possible reason for this, which may also explain why menstrual synchrony happens. In one study, sweat from some women’s armpits, taken at various times in their cycles, was applied to the upper lips of other women, three times a week for five months or longer. Most of the recipients were menstruating in sync with their donors by the end of the study.



Author: Leslie Carol Botha

Author, publisher, radio talk show host and internationally recognized expert on women's hormone cycles. Social/political activist on Gardasil the HPV vaccine for adolescent girls. Co-author of "Understanding Your Mood, Mind and Hormone Cycle." Honorary advisory board member for the Foundation for the Study of Cycles and member of the Society for Menstrual Cycle Research.