Do our periods actually sync?

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Are your female friends and family always moody during the same time of the month as you? Everyone reaching for a tampon or pad at the same time? Do you think your menstrual cycles are sinking? What does Science have to say about this?

The menstrual cycle

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The length of our menstrual cycles are determined by various factors such as our age, diet, stress levels and birth control methods. In our teen years, a normal menstrual cycle length is usually between 21-45 days, with an average of 28 days, but may sometimes be longer or shorter. Over the one or two weeks leading up to a women’s period, many experience symptoms such as cramps, food cravings and mood swings. Women who claim to have “Synced” menstrual cycles often experience these at the same time as those they are “synced” with.

What research has been done so far?

To further investigate this phenomenon, in 1971, Dr Marth McClintock conducted a study based off 135 women living in the same college dorm. Her research found that over the course of the school year, there was a higher percentage of period synchronization for roommates and close friends, compared to random pairings of women. Her hypothesis was that this synchronization could have been due to the time, meals and stress that close friends experienced together. This could be very relevant, considering the similar factors that would hence go into influencing their menstrual cycles. Another proposal of hers was that the awareness of another woman’s period, could cause the pheromones of women to communicate with each other due to physical closeness, hence triggering their cycles to sync. However, there have been several other researchers that have found fault with her theory and experiment.

Mixed findings

A study in 2006 collected data from the menstrual cycles of 186 Chinese women living in a college dorm together, with the results showing that their cycles did not sync up. In addition to this, Beverly Strassman, a professor of anthropology at the University of Michigan argued in her 1999 paper “Human reproduction” that the so called “Synchronization” of women’s menstrual cycles was a probability that was bound to happen, and was not related to their close contact. She argues that if the menstrual cycle is 28 days long, 14 days would be the maximum amount of asynchrony of the menstrual cycles of 2 women. Statistically, this would mean that the average asynchrony of the menstrual cycles of 2 women would be 7 days. As a result, given that menstruation lasts on average 5 days, overlapping periods are a common occurrence, and are not a result of close friendships or contact. This could mean that period overlapping is just explained by the law of probability. The syncing of your menstrual cycle with your friends and family could just be by chance!

Theories that support the syncing of menstrual cycles

Currently, the most prevalent theory is that it is a result of evolution, a strategy among females to avoid the same male impregnating all of them at the same time. The concept was that if women in the same circles had synchronised cycles, they would all be fertile simultaneously. Thus, the same man would not be able to reproduce with them all as he would be unable to manipulate all the females at the same time. If this is indeed true, I think it really interesting and amazing how evolution has helped females not to be taken advantage of in this way!


It’s in human nature to connect our physical experiences and frustrations like our menstrual cycle to emotional ones. Having a menstrual cycle that “Syncs” with a close friend might seem as though you have an extra special and close friendship. However, it is also extremely important to remember that just because your period does not sync with the women in your life does not mean there is anything wrong with you or your relationships.

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As with many women specific health issues , menstrual synchrony lacks much research. It definitely deserves more attention and time from researchers, regardless of how challenging it may be to prove or disprove the theory. There has not been a large range of research in the area recently and there is a great potential for more to be discovered. Until the day it is totally disproven, the discussions of syncing menstrual cycles will continue to live on and my friends and I can continue to be grumpy and have cravings for food together each month!


1. (2020). Retrieved 24 October 2020, from

2. Is it true that periods synchronise when women live together?. (2020). Retrieved 24 October 2020, from

3. Is Period Syncing a Real Thing? Why Women’s Periods May Sync Up. (2020). Retrieved 24 October 2020, from

4. Stages of Menstrual Cycle: Menstruation, Ovulation, Hormones, Mor. (2020). Retrieved 24 October 2020, from

5. What’s "normal" menstrual cycle length and variation?. (2020). Retrieved 24 October 2020, from's-normal-menstrual-cycle-length-and-variation

6. Yang, Z., & Schank, J. (2006). Women do not synchronize their menstrual cycles. Retrieved 24 October 2020, from


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