Menstrual Synchrony: Do Girls Who Go Together Flow Together?

When women live together, do their menstrual cycles tend to synchronize? It’s been a long time since I first heard that claim. I didn’t believe it, for a number of reasons. I had never observed it myself, I saw no plausible mechanism to explain how it could happen, I thought the statistics to prove it would be problematic and complicated, and I suspected that confirmation bias and selective memory might have persuaded people that a spurious correlation existed. How often do women say “Oh, look! We’re having our periods at the same time”? How often do they say “Oh, look! We’re having our periods at different times”?  Now that many years have passed since my first encounter, I thought it would be fun to revisit the claim and see whether science has supported it or rejected it.

A perusal of PubMed and other Internet sources left me confused and amused.

Synchrony Is Difficult to Define

Consider that the normal menstrual cycle can vary from 21 to 35 days and can last 2 to 7 days. Consider that some women are regular and consistent, while others have variable patterns, even “regularly irregular” patterns. Consider that anovulatory cycles and other conditions often lead to menstrual irregularities that fall outside the normal range. Consider that strenuous exercise and other life events can affect menstruation. Put all that together, and you can see that often cycles will overlap simply by chance, and that it is difficult to define synchrony.

If two women have regular 28 day cycles and 7 day periods, the maximum number of days they could not overlap is 14. On average, their periods will be 7 days apart, and half the time they will be closer.

How could a 21 day cycle ever “synchronize” with a 35 day cycle? For example if you compare a woman with a regular 35 day cycle who starts on January 1 to a woman with a 21 day cycle who starts two weeks later on January 15, their next periods will coincide almost perfectly (Feb 4-10 and Feb 5-11) but they will diverge after that. Would it count if the last day of one woman’s period overlapped with the first day of another woman’s? What if half the periods coincide and half don’t? The whole thing is problematic.

What Does the Literature Say?

It all started with Martha McClintock. In a paper published in Nature in 1971 she found that “social interaction” in a college dormitory setting could have a strong effect on the menstrual cycle. A follow-up study in 1998 tended to support the hypothesis that pheromones were involved: smelling armpit secretions of other women could either lengthen or shorten cycles depending on what part of her cycle the donor was in.

I’ll summarize rather than trying to cover everything published on the subject.  A Scientific American article did a good job of reviewing the literature as of 2007. Suffice it to say that about half the published papers support the synchronization hypothesis and half don’t; and the half that do have been harshly criticized for their poor design and poor statistical analyses. So we haven’t reached a consensus, but it’s looking more likely that synchronization is a myth.

A study in a nursing journal assumes that synchronization occurs and addresses the subjective meaning of the experience to

assist nurses to understand the holistic aspects of this everyday experience of women and to design effective strategies and techniques to help women gain knowledge about their cycle functions, promote healthy attitudes toward menstruation as a process, and acknowledge and honor this natural, healthy aspect of their menstrual cycle.

I will be kind to those nurses and apply Thumper’s rule. (In Bambi, he said “If you can’t say something nice, don’t say nothing at all.”)

What About Non-Humans?

It was originally thought that other primates and other mammals demonstrated menstrual synchrony due to pheromones, but recent studies have shown that it doesn’t occur in chimpanzeeshamsters, mandrills, or golden tamarins.

Do Human Pheromones Exist?

The existence of human pheromones is controversial. Pheromones are chemical signals released by one individual that affect the behavior of another individual of the same species. They can be useful for alarms, food trails, sex attractants, and other purposes. They have been well documented in insects and even in plants. In one memorable demonstration, a drop of oleic acid was all it took to persuade other ants that a live ant was dead; they dragged it back to the trash heap over and over, ignoring the fact that it kept protesting its trips to the ant graveyard by vigorously kicking, and that it kept “coming back from the dead.”

In humans, pheromones have been postulated and even sold as sexual attractants. But there is little or no peer-reviewed evidence to suggest that any pheromone influences human behavior. No human pheromones have been identified, and the vomeronasal organ that detects pheromones in other mammals is rudimentary and nonfunctional in humans.

Other Explanations

Not knowing if it occurs hasn’t stopped people from speculating about why it occurs or about why it doesn’t. The evolutionary reasoners have chimed in with just-so stories both about why it would and wouldn’t offer a survival benefit. For instance, if everyone ovulated at the same time, it would be harder for a woman to get a man. Or being banished to the menstrual hut at the same time would allow women to bond and collaborate on social enterprises. I was particularly amused by one chain of reasoning:

Synchronization also doesn’t make sense because during their mentrual cycles, women often experience lack of energy. These spells of low energy among women in a tribe would be detrimental to a hunter-gatherer lifestyle. Furthermore, if all of the women get pregnant at the same time, they won’t be able to help each other. Given our anthropological history, evolution should select that women do not cycle together. (errors in the original)

And this comment on the Scientific American article left me rolling on the floor:

Astrology explains this for better than modern science. Normally close friends have similar sun, moon positions in their horoscopes. The moon and mars determine the menstrual cycles. Astrological harmony makes them friends than living together which in turn is the cause of synchronization. Even if they are living far in different continents they will be in sync. In India any normal astrologer can check a woman’s horoscope and can tell the date of her menstruation time ( especially used when they have to attend auspicious ceremonies) and plan the events. (errors in the original)


Do women’s menstrual cycles tend to synchronize when they live together? As so often in medicine, science doesn’t provide a clear answer. Even if the phenomenon occurs, and even if human pheromones cause it, so what? It’s a matter of curiosity, not of clinical importance.  When there is a reason to manipulate ovulation or menstrual timing, pharmaceutical hormones work very well. The paucity of evidence for human pheromones suggests that if they do exist, their effects must be too small in magnitude to be very important.

Posted in: Obstetrics & gynecology

Leave a Comment (25) ↓

25 thoughts on “Menstrual Synchrony: Do Girls Who Go Together Flow Together?

  1. Al Morrison says:

    No agreed upon definition of menstrual synchronization + no plausible mechanism + poor research supporting the hypothesis + split research results + no support from comparative research + a tendency to over report synchronization = MYTH.

  2. superdave says:

    Amazing that even the proposed mechanism of this myth, is probably also a myth. When you got a myth^2, its not a good sign.

  3. windriven says:

    It is hard to fathom an evolutionary advantage to menstrual synchronicity. What is gained by all females being in estrus together? Tired males? What is gained by all females menstruating together? Time for all the males to go on a fishing trip?

  4. Diomedes says:

    Just more evidence that evolutionary biology, normally a very rigorous field, loses all that rigor when discussing the evolution of human behavior.

    Actually, it’s usually not the same people. There’s a whole crew of “evolutionary psychologists” that spend all their time arm chair philosophizing, theorizing, and not assembling rigorously collected data. They’re not generally held in high regard by evolutionary biologists. I’m surprised, and disappointed, that Barbara McClintock was involved in this particular exercise.

  5. wbgleason says:

    NOT Barbara McClintock! Please fix…

  6. dogmatichaos says:

    I think some of the “anecdotal evidence” used to support this comes from observations in behavior rather than menses. Women, like any other group, tend to have behavior patterns that correlate strongly with those around them. The more they’re around, the more they become “in-tune” with each other and respond to each others emotional states.

    Of course, the conventional knowledge is that women become agitated or more emotional during “that time of month.” Naturally, when a person sees a group of women that spend a lot of time together who’s emotional states are very similar, he or she may erroneously ascribe those synchronized emotional states as having a hormonal basis rather than a behavioral one.

    That’s just my two-cents.


    NOT Barbara McClintock! Please fix…

    Right: it was not Barbara McClintock. Fixed it.

  8. LovleAnjel says:

    Two thoughts:

    There doesn’t need to be an evolutionary advantage. There just needs to be not much of a disadvantage.

    A lot of women are on hormonal birth control, and the given starting day is Sunday. Most women probably choose to start that first pack on an easily remembered date, like around the first of the month. Voila! They are synchronized.

  9. Diomedes says:

    “Right: it was not Barbara McClintock. Fixed it.”


  10. Amy (T) says:

    when I was a teenager and played basketball with a group of girls, our moms told us about “synchronizing” and I believed it at the time. I thought about it a few years ago, and thought it didn’t sound very plausible and wondered what truth there was behind it. Looking back, we were early-mid teens, only having started our periods within the last couple years, we were very active, and I know I had a fairly irregular, but longer cycle, and with 11 girls, it’s no stretch that a chunk are going to be menstruating at a given time.

    LoveAnjel:”A lot of women are on hormonal birth control, and the given starting day is Sunday”

    although the period week starts Sunday, I never started then, mine were much later in the week, overlapping with starting the next month’s pill. also, if the study was done in 1971, the myth was probably around well before then, meaning it probably originated before women were on birth control (and I hope any study would take BC into account); but today women could certainly think that without considering the BC effect.

  11. This article was quite amusing! Thank you! I will enjoy sharing with my female friends who believe in astrology, pheromones, and synchronicity of periods.

    Admittedly, I am a bit embarrassed to admit that I did not know that human pheromones were also a myth and had not been identified. To think of how many times in my adult life I attributed an interpersonal phenomenon to “pheromones.”


  12. What explanation would there be: pheromones, psychic telepathy, synchronized mind over body/matter vitalism?


    Uh, this makes no sense whatsoever as non- synchronized cycles would make only a difference of +- a few weeks in conception timing for pregnancies that last ~9 months. Non-synchronized cycles do not prevent simultaneous pregnancies.

  13. ConspicuousCarl says:

    It is clearly a matter of lunar influence, not synchronization with other humans, as 95% of menstrual cycles begin within one week of either a new or full moon.

    (concept stolen from someone, possibly Michael Shermer)

  14. cedge20 says:

    I enjoyed this article a lot, too. I’d heard about pheromones and synchronicity in college (30+ years ago), lived in a house with 7 women so of course there was some common experience there. I know the myth lingers; I’ve a friend who’s ending her menses who avoids anyone menstruating for this very reason….

    Love this site. I’m a non-science major (as they used to call us) and turn to y’all for sensibility.

  15. lilady says:

    I never believed in that old wives tale of synchronization. Back in the 1950s growing up with my sister, we would often argue about who used the last menstrual pad and who was responsible for schlepping to the drugstore to re-stock the supply.

    Even then, I could not understand why you could only purchase sanitary pads in the drugstore and they were stocked out of sight behind the counter….boxed in cardboard with cheap pink and white wrapping paper. Of course, everyone knew what you had just purchased in spite of the ruse.

    Dr. Hall, you cited a nursing journal article which was from a holistic nursing journal describing a very unscientific study. The majority of nurses do not plug into “holistic” nursing…we think such nursing “practice” as a blot on our profession.

  16. Jan Willem Nienhuys says:


    If ‘helping each other’ refers to helping with delivery, this is also nonsense, even if all those women got pregnant in the same cycle (which need not be the case), because pregnancies on average are about nine months, but there is a lot of variation.

    Moreover, in the olden days, women possibly usually got pregnant after they stopped nursing a child, without any period to be synchronised. Of course, we weren’t there so we don’t really know what went on half a million years ago.

  17. DW says:

    dogmatichaos wrote:


    Um … wait, is there any evidence for THIS?
    Is this supposed to be in comparison with men? It does seem to me that humans respond to each other’s emotional states, but that’s just a truism. Stated as something particular to women, my “sexist BS”-ometer is going off wildly.

  18. KB says:

    Wow, I’ve always thought menstrual synchrony was true, despite living in college dorms and noticing that it didn’t happen. (I thought my friends and I just weren’t around each other enough, since we left the dorms to go to classes and eat and whatnot.) I am less skeptical than I thought.

  19. Simon says:

    As a vet the idea of menstrual synchronicity is not too alien to me as it is so common in the animal world. However, this only occurs in animals that are seasonal breeders. In these animals it is easy to synchronise their coming into oestrus by eg. altering day length with artificial lighting to make them think Spring is starting. In pigs, you can bring one into oestrus using a rag soaked in boar sweat. Therefore, I doubt that woman can cause others to synchronise with them, but it does seem feasible that external environmental factors could affect a whole group to bring them into season (sorry, vet terminology dies hard- no offence intended!) at the same time, though obviously if there is no evidence that such synchronicity exists then such speculation is somewhat moot.

    If we’re going to play the evolutionary psychology game then here’s my guess- Imagine a pack of proto-humans. If oestrus is synchronised then a number of males will service a number of females over a short period. The young will, therefore, be born over a short period in a cluster. The males then cannot be sure which of the young are theirs as opposed to their rivals. Thus, the best strategy for defending his genes is to contribute (or at least not impede) the welfare of all of the young in the pack, leading to a mutually beneficial society which could be very evolutionarily advantageous.

    Where I suspect this hypothesis would fail is that humans are polyoestrus and not seasonal breeders.

  20. Calli Arcale says:



    Actually, there are three problems with your assumption that most women start that first pack on an easily remembered day.

    First of all, starting it on the first of the month has no real advantage; the typical regimen lasts 28 days, and there is only one month which is 28 days long (excluding leap years, when there are no months which are 28 days long). So the date will creep away from the first of the month very quickly. The packs are marked for Sunday (though this is optional) merely because Sunday will be an easily remembered day of the *week*. (The medication actually changes each week; there are three sets of pills with different levels of hormones in them, and then one set of sugar pills for those ladies who want to make sure they don’t get used to skipping a pill and miss starting the next pack in time.)

    Second, women don’t really get to pick what day to start it on, exactly; you can fudge it by up to a week (they only suggest Sunday for convenience; you could as easily pick Wednesday), but ultimately it’s your body that decides. You have to start the pills during your period. The instructions usually suggest the first Sunday after your period starts, and the sooner the better to ensure protection during that first cycle. So starting it at the beginning of a month will never be an option, though it may occur through chance.

    I forget what 3 was going to be…..need more coffee.

  21. Wow, it sure is easier to collect skin surface pheromones passed in kissing than to collect ultra-trace amounts of imperceptible odors. I don’t understand why my tiny-minded colleagues leave this wealthy mine only to me. It only takes two days to collect enough pheromone to profoundly change someone’s life. Losing criminal inclinations or ditching a heroin habit in a few minutes can be life-changing. I’m getting sick of all the testimonials and anecdotes piling up around me. I will be glad to show any researcher how to cure drug addiction (heroin or cocaine), delinquency, running away, or profound sexual perversions like unwanted homosexuality in an adult, child molestation/pedophilia, fetishes, all these can be miseries of the past.

    A broad-spectrum  medical treatment for thrill-seeking (crime, drug addiction, unwanted perversions) now exists: a human pheromone, the healthy adult male facial skin surface lipid ‘kissing daddy’s face’ pheromone.  Perhaps due to differing metabolic/neuronal pathways, alcoholism is unaffected by pheromone treatment.  One dose of 150-250 mg provides permanent relief of even the most obdurate cases.  


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    Nicholson, B. 2011: Exocrinology The Science of Love 2nd Edition Human Pheromones in Criminology, Psychiatry, and Medicine.

    BBC-TV interview
    typical anecdote

    Read a preview OF LOVE 2nd edition Textbook of Medical Science: Exocrinology
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  22. peeramid says:

    There is too much heat (as in entropy) in the preceding column, but too little light. Numerous scientific studies have shown that women’s menstral cycles are influenced by the phase of the moon when women live outdoors. Further, fertility for women living outdoors depends on how closely their ovulation matches the timing of the full moon. This is because circadian rhythms are controlled by the pineal body, a part of the brain which is derived from optic tissue.

    It is entirely possible that sorority sisters would ovulate together, provided the late-night light sources for their carousing varied by the day of the month. Or in general, women living together and experiencing the same monthly variation in lighting would synchronize their periods accordingly.

    Every finding from the above can be explained this way. A scientific study with inconclusive results probably asked the wrong question. Above, none of the cited studies considered questions of monthly variation in circadian light sources.

    1. Harriet Hall says:

      Studies of an alleged correlation between menstrual cycles and lunar cycles are prone to the same flaws as the studies cited. And I think the lunar mechanism is far less plausible than pheromones.

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