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Do Women’s Cycles Sync Up?

By Jane Harrison-Hohner, RN, RNPOctober 4, 2010
From the WebMD Archives

In 1971, Martha McClintock, a young graduate student, published an article called “Menstrual Synchrony and Suppression” in the science journal Nature. Her study population was the women in her dormitory. Her findings stated that over time, women who lived together tended to cycle together.

Since that time, the subject has expanded to include studies about the role of pheromones as triggers for menstrual synchrony. So as many of our readers are returning to college, and many have female housemates, let’s take a look at menstruation as a group process.

What is menstrual synchrony?

Loosely defined, menstrual synchrony means that women’s cycles “sync up” so that roommates, dorm residents, bunk mates in the military, or mothers and daughters begin to adjust cycle lengths such that females begin to be on their periods at the same time.

Some reported adjustments in cycles included: two days closer together over a four to six-month interval (McClintock, 1971) and a 1.4 day shortening (Stern & McClintock, 1998). This is not a dramatic change.

What conditions would lead women to synchronize?

There have been studies looking at many types of female proximity. Menstrual synchrony was reportedly identified among a variety of female groupings: college roommates (McClintock 1971), lesbian couples (Weller& Weller, 1992), roommates who are close friends (Weller, 1995), and sexually segregated Bedouin women (Weller & Weller, 1997). Yet menstrual synchrony was not found among: roommates who are not close friends (Weller, 1995), nor lesbian couples living together for an extended period of time. (Weller & Weller, 1998).

As you can see, the same group of researchers were not able to duplicate findings of menstrual synchrony among groups that were very similar (e.g. lesbian couples were synchronized, whereas lesbian couples who had lived together a mean of three years were not synchronized). One postulated reason for this is that apparent synchrony is really just due to chance.

Two of the most recent studies have strengthened the idea that the best condition for cycling together is chance. Kiomkiewicz and colleagues (2006) evaluated 99 women for five months and found no synchrony of menstruation. The longest study, lasting one full year, assessed 186 Chinese women living together in dorms (Schank & Yang, 2006). Again, no menstrual synchrony emerged.

Why would it appear that women cycle together?

Readers of this blog and members of our Women’s Health Community are likely aware that women’s cycles have variable lengths. A “normal” cycle length is considered to be anywhere between 21 and 35 days, and most menstrual flows last between five and seven days. Thus, in a group of women, it would not be uncommon for the bleeding days to coincide at some point.

There is often variability in cycle lengths in the same woman. Women with missed or erratic ovulations likely have the most variability. Regular readers of our posts have surely seen the number of women who write with concern about their periods being a few days early or late. Community members may also remember reading posts about menstrual periods suddenly moving from the second week of the month to the third (or vice versa).

Among the published studies of synchrony, there can be issues with choice of statistics or methods of study as well. For example, results could be influenced if a researcher did not have baseline data to identify erratically cycling women, included women using a hormonal form of birth control, or merely asked women about the first day of their last period (instead of having a written record).

Within the past decade, one of the other explanations for contradictory findings between menstrual synchrony studies has been the ability to smell pheromones. A pheromone is a chemical substance excreted where the odor of the pheromone gives a subtle message to another member of the same species. In pigs, the male boar’s saliva has two pheromones which cause the receptive female pig to not evade his advances (Perry, 1980). The same two pheromones are present in human underarm secretions. One of these pheromones (5 alpha-androstenol) is purported to be linked to menstrual synchrony.

Morofushi and colleagues (2000) evaluated 64 Japanese women living in a college dormitory. Twenty-four young women became synchronized with a roommate within three months. After assessing all variables likely to impact menstrual timing (including degree of emotional closeness) the most predictive element was the ability to smell 5 alpha-androstenol. Interestingly, in the cases of synchrony, both members of a pair had to alter their cycles to achieve the same time of flow.

But before you become concerned about your roommate’s sweaty gym clothes draped all over the room, consider the evidence presented by Jahanfar (2007). A group of 88 students sharing housing were checked for similar menstrual timing and the ability to smell 5 alpha-androstenol. Almost 60 percent of the women did have similar cycle patterns, but there was no association with the ability to smell the pheromone.

Women cycling together over time is an idea I would really like to believe, but the studies (even by the same researchers) are not very consistent. Perhaps we are actually better off if roommates are not having their own PMS stress-outs at the same time!

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