Menstrual Synchrony

A well-known study on menstrual synchrony was published by Martha McClintock, an American psychologist who based her search on a group of women living in the dormitories at Wellesley College of liberal arts in the state of Massachusetts, USA. She hypothesized that pheromones were at the base of menstrual cycle synchronization. She focuses on the relationship that the environment and biology have upon sexual behavior. Other mechanisms have also been postulated especially with the lunar cycles.

More than a year after the pandemic due to the virus SARS-CoV-2, women’ menstrual cycles have seen changes among the one leaving in the same household. A new survey published in the Guardian reports that a majority of menstruating women have experimented changes in their cycles over the past year. Stresses and changes in their lifestyle are certainly to blame. We are well aware of the facts that the symptoms of post-menopausal women get worse with their life changing because of the constraints imposed by the pandemic. Most women have noticed changes.

I was approached by some close friends who lived together, with their daughters and daughter-in-law during this time and they were puzzled by the sudden changes in their hormonal cycles and expected from me an explanation to the apparent problem. Indeed, almost ¾ of women during the pandemic has noticed such changes. They wanted to have an understanding on the fact that all of them in the household started menstruating near or almost at the same time

In another study, a sport scientist found out that 55% of athletes have reported changes to their cycles. Job security, and other stresses on the daily living activities accounted for the problem and the menstrual periods were seen more often with shorter cycles or were concomitant with other women living in the same house, presenting in different timing. The article reported changes in the estrogen and progesterone cycles in their body with also an increase in their bleeding time. Persistent stresses can release cortisol and the overproduction of cortisol can suppress the normal physiologic level of the reproductive hormones in the body.

The lack of entertainment and the absence of friends around can build up stress and affect the psychological wellbeing. There are no distractions and no coping mechanisms and this will affect the hormone level and the menstrual cycles. A psychological element directly related to the lockdown will certainly affect the body. The study of Martha McClintock paralleled the problem seen with the COVID-19 seclusion.

I reproduced a women’s string figure depicted in the Yolngu people mythology on menstrual synchrony demonstrating the menstrual blood stream of three women. This alleged phenomenon of Menstrual Synchrony is apparently seen when women who begun living together in a close proximity, experience a menstrual cycle onset becoming synchronized in time… An example of seven lifeguard swimmers living together at the beginning of a summer to facilitate their training exercises, realized that after passing three months together that the onset of their menstrual cycles fell in a 4-day period.                               

A women’s string figure found in the Yolngu people mythology on menstrual synchrony.

The Yolngu or Yolnu are aboriginal people inhabiting in the northern territories of Australia. The term Yolngu mean person in their language. They were fisherman trading goods with the south Australian governments and it is believed that they may have had contact with merchants in China as well.

The article of Marthe McClintock was published in Nature Magazine in the 70’s, stating that menstrual cycle synchronization happens when two or more women become closer in time together. Unfortunately, many other papers have contradicted the findings of Martha McClintock. The recent COVID-19 pandemic may have risen suspicions in my mind because of two groups of women, well known to me who have realized that the new way of living home, in a seclusion, has bought again in question this phenomenon and created more doubts in our mind.

Two family of 3 and 4 female members have experienced menstrual synchrony and by curiosity, I have taken the liberty to explore the topic for our AMHE passionate readers. I have also questioned at least ten (10) Obstetricians and gynecologists belonging to our association, the AMHE to have their impressions on the subject. Some heard about the phenomenon but at my surprise, many did not know about the experience of Marthe McClintock. One fact for me is sure that 2 families who approached me, remained convinced that the seclusion and the proximity of the women involved, have implied drastically, changes in their menstrual period cycles. Let us review the literature about the topic.

Martha McClintock hypothesized that the pheromones could cause menstrual cycle synchronization but unfortunately no scientific evidences have supported the theory. The same was advanced with a lunar hypothesis playing a role in the change of women cycles but doubts still remain on the pheromone mechanism. If many have criticized the findings, others have reported their failure to find any menstrual synchrony and even have criticized the method used to search for a cause justifying menstrual synchrony. The term “synchrony” itself appears to bring confusion and should not be used for such phenomenon because it is already used in the description of cycles becoming closer to each other over time. Harris and Vitzthum have reported more doubt than acceptance of the hypothesis.

McClintock suggested also that this synchrony may be an epiphenomenal lacking biological function because this is well known that ecologists have already studied it on animals in the wild. No matter if it is seasonal, tidal, lunar, this is a general phenomenon of the reproductive function relatively common in the co-cycling females to attract the males during breathing season. Inversely, if too many females are cycling together, they will be competing for the quality males. The question remains that women who lived together do in fact synchronize their menstrual periods. The ideas also that menstruation maybe in harmony with the cosmic rhythm has surfaced in many myths and rituals in the traditional communities around the world.

A story of two sisters, the Wawalik sisters, at Mudawa near Buckingham Bay explains that:  two sisters sat looking at each other with their feet out and their legs apart and both started menstruating. Another more recent story related to me by one of our Obstetrician and Gynecologist (PD) of the AMHE, revealed that while he was a physician-witness of two twin sisters who came together for a consultation with him at a hospital in New-York city, because one was having a miscarriage and while aborting the fetus, suddenly the other twin started menstruating as well. In the northern California, there is a story of the Yurok people relating on a household of fertile women not pregnant at the time, but who menstruated together at the same time. Another anecdotal story tells that if a woman sees traces of menstrual blood on another woman’s leg or even if she is simply told that another woman has her menstrual period, she will begin menstruating as well. A story has the Yurok women who were practicing bathing rituals together by “talking to the moon”, in order to menstruate together. I can go on and on.

This brings us back to the phenomenon described by Martha McClintock, the psychologist who described the phenomenon of “menstrual synchrony” on the young women living in the dormitory at the Wellesley College in the 1970’s. Many studies have tried but failed to duplicate the observations noted in her essay: 135 young women between the age of 17 and 22 were part of the study. They were all residents at the same dormitory. The women were asked about their menstrual period and their relations with men as well. McClintock found out that the one associated with men presented a shorter menstrual cycle.  But the other group synchronized their menstrual periods like the effect produced by the pheromone on laboratory mice (Whitten effect). Let me bring you up to date on some experiences performed since, trying to duplicate or disprove the McClintock theory.

In the 1980’s, Graham and McGrew were the first to replicate McClintock study using 79 women in a college in Scotland. They were aged 17 to 21. She replicated the study proving that the one who were close friends synchronized their cycles and she considered a pheromone as the cause but added that emotional attachment may have played a role. Guadagno and al replicated as well the experience of the McClintock’s study on 85 women, sorority sisters living in dormitories who attended an American midwestern university. Two (2) to four (4) were living together. The study concluded that the women synchronized their menstrual cycles. They also advanced that the pheromones may have played a role.

A third attempt at using college students was done by Jarrett and 144 women between the age of 17 and 22 attending two different colleges, participated in the study. They were all, paired as roommates. The women did not synchronize and they conclude that the women had more irregular menstrual periods.

In the 1990’s, Wilson, Gravel and Kiefhabe performed two studies on two groups of college students. A group of 132 at the University of Missouri between 18 and 22 of age and another group of 24 women between the age of 18 and 31. At the conclusion, they found no synchrony in either study.

Other studies were carried on lesbian couples. In a first group of 20 couples between the age of 19 and 34, Weller and Weller concluded that more than half of the couples had synchronized within 2 days timespan of each other cycle. They repeated the study in a Bedouin village, in Northern Israel on 27 families which had from two to seven sisters aged 13 and older. They collected data for a 3 months period. They concluded that menstrual synchrony occurred in the first two months period but not for the third month for the roommate sisters but not for the families as a whole. A second study on 29 lesbian couples between the age of 22 and 48 carried by Trevathan, Burleson and Gregory found no evidence of synchrony.

122 women from a Dogon village were studied by Strassmann to know if menstrual synchrony was encountered among their fertile population of 3 to 8.6 life birth per women with a median cycle of 30 days. She did not find any evidence of synchronization and concluded that menstrual synchrony was adaptive.

In the new millennium other studies were carried out. Yang and Schank conducted a study on 186 Chinese college students and divided them in two groups. 93 women lived in 13 dorms where 5 to 8 women were sharing the rooms and 93 others were placed in 16 dorms where 4 to 8 women were sharing 29 rooms. Each woman was given a notebook to record the onset of their menstrual periods and data were collected over a year. The cycles were of 28 or 31 days in length. They found that in 9 of the 29 groups, women cycles converged for one cycle closer than expected but they concluded that there was no evidence the women in these studies synchronized their menstrual cycles.

Ziomkiewicz conducted another study on 99 Polish college women sharing two dormitories. 36 women lived in 18 double rooms and 61 lived in 21 triple rooms, all recorded their menstrual period onsets on a calendar and data were collected for 6 months. The mean menstrual cycle was 30.5 days. No statistical difference was found and concluded that there was no evidence of menstrual synchrony.

In fact, many like Clyde Wilson argue that McClintock did bot correctly calculate the initial onset difference among women and concluded that the study was biased towards asynchrony. Yang and Schank followed up by using computer simulations to estimate the average onset and reported a 5 days difference in the cycles while with McClintock the average was 6.5 days. They concluded that if their analysis was correct, it implies that synchrony did not occur in McClintock study.

Cutler and Law reviewed the lunar hypothesis and did not agree on any phase where the lunar menstrual cycles synchronize either. Strassmann reported on the Dogon women who were living outdoors most of the nights with no electrical lightning. They would have been ideal to detect a lunar influence on menstrual cycles. She found out no relationship. Jarett found that women with a need for social recognition and approval from others were associated with synchrony.

The oscillator hypothesis of the pheromones speculated by McClintock able to cause menstrual synchrony, was also challenged first on Norway rats and then extended to humans. It was proposed that human females release and receive pheromones that regulate the length of their menstrual periods without consciously detecting any odor. Specimen were collected under the axillae at different phases of the women menstrual cycle: Follicular, Ovulatory and Luteal phases) were taking from donor women and the applied under the nose of recipient women. Often, the donor woman will wear cotton pads in their underarms for around 8 hours. These pads were then frozen to prevent the smell (scent). Odorless compounds collected during the time of ovulation triggered a hormonal event in the recipient that lengthened their menstrual cycles. Stern and McClintock concluded that these findings proved the existence of human pheromones.

Arden and Dye prepared a 4-page questionnaire and sent it to 122 women at Leeds’s University to know if they were aware of menstrual synchrony. The synchrony was described to them like when 2 or more women who spent time with each other and have noticed their periods almost at the same time. They were asked if they knew the fact and whether they experienced it. They were asked for details and the frequency and how long the periods lasted. They found that 84% of women knew about the phenomenon but only 70% reported it with considerable variation. They concluded that the synchrony was variable but an objective phenomenon.

Compounds were identified as pheromones (3 and 5 alpha androsterones) in the relationship between menstrual synchrony and the ability to smell the putative pheromones in 64 Japanese young women living in together in a college dormitory. One-third of them became synchronized with room-mates in a period of 3 months. It was assumed that the pheromones that mediate the synchrony were detected by the olfactory system as shown in some animals by Martin et al 1986 and Dories et al, 1997. It is possible also that this pheromonal effect may have been mediated by the accessory olfactory system as well. It seems, then that synchrony between two women could be achieved by either one or both, shifting their cycles.

Wilson, Arden and Dye certainly understood that the phenomenon of Menstrual Synchrony can be seen by chance when menstrual variability is observed. Cycles can be irregular in frequency without having anything to do with the pheromones. By example, if two women with menstrual periods different in length become close friends, the onset of their menstrual cycle onset will progressively adjust until they coincide or by example If one friend has a 14-days in cycle onset, the other friend can easily adjust by two or three days until both cycles coincide. The other scenario is that the cycle can converge or diverge creating the appearance of synchrony during convergence. This is why the phenomenon may last for a number of months. Some may define menstrual synchrony as a menstruation overlap.

Female competing for good genes should avoid ovulatory synchrony because of behavioral ecology. It is best seen among primates, in a particular female spatio-temporal distribution, affecting the ability of any single male to monopolize mating. More females are fertile at one time and all can’t be impregnated by only one male. A female in need of a male will synchronize her cycle. A dominant male can maintain his monopoly only if his females stagger their fertile periods so he can impregnate them at a time.

Synchronism may have been a factor during the evolution whether it is seasonal or lunar. Infant mortality may disrupt synchrony while it emerges that reproductive synchrony can be an effective women strategy to undermine primate-style sexual monopolization by dominant males. The controversy remains then unresolved.

Estrous synchrony is similar to menstrual synchrony and has been reported in other mammalian species like rats, hamsters, Chimpanzees, etc. Menstrual synchrony is  certainly a known process (McClintock effect) but also women who live together or in close proximity involuntarily synchronize as well their menstrual cycles with each other. The original data from the original studies may have been incorrect because the effect could not be replicated.

In conclusion, many women still believe that their cycles are influenced by life style. It is more apparent to the one suffering from PMS or pain syndrome with their menstruation. If the original papers did not support the McClintock effect, Martha McClintock further offered intriguing evidence of the existence of pheromones causing the menstrual synchronism. This was pointed as well in other studies that men as well produce these pheromones which are able to influence menstrual cycles. There is still no convincing evidence of the existence of these pheromones. Science seems to be inconclusive on whether menstrual synchrony is a true phenomenon or just a belief that has reached the status of urban legend. No matter. It is a fact that has become accepted by most women.

I hope that I have shown enough competence to answer some questions to the one who approached me with their doubts on menstrual synchrony. To my colleagues OBGYN friends who never heard about the phenomenon, I feel like I have opened a pandora box that will trigger plenty more discussions on the subject. 


1-    Gosine, Anna: “Do Women Who Live Together Menstruate Together?”. Scientific American (December 7, 2007).

2-    Strassmann I: (1999) “Menstrual synchrony pheromones: cause for doubt”. Human Reproduction. 14(3): 579-580.

3-    McClintock, M. K. (1971). “Menstrual Synchrony and Suppression”. Nature 229 (5282) pp 244

4-    Trevathan, Wenda R, Burleson Mary H, Gregory, W Larry (1993). No evidence of menstrual synchrony in lesbian couples: Psych neuroendocrinology: 18 (5-6) pp 425-435.

5-    Harris, Amy L, Vitzthum, Virginia J (2013). “Darwin’s Legacy: An Evolutionary View of Women’s Reproductive and sexual Functioning”. Journal of Sex Research: 50 (3-4): 207-246.

6-    Ziomkiewicz, Anna (2006): “Menstrual synchrony: Fact or Artifact?”: Human Nature: 174): 419-432.

7-    Hall, Harnett (Sept 6, 2011). Menstrual Synchrony: “Do Girls Who Go Together Flow Together?” Science -Based Medicine.

8-    Adams, Cecil (2002-12-20): “Does Menstrual synchrony really exist?”. The Straight dope.

9-    Menstrual Synchrony between mothers and daughters and between roommates”. Psychology and Behavior: 53 (5): 943-949.

10-Weller, Leopold. Weller, Aron, Roizman, Shoshana (1999). Human menstrual Synchrony in families and among close friends: examining the importance of mutual exposure”. Journal of Comparative Psychology. 113 (3): 261-268

12- Whitten Wes (1999). Pheromones and Regulation of Ovulation” Nature. 401 (6750) pp 232-233.


Maxime Coles MD
Boca Raton FL

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