Myth or Fact: Women’s Menstrual Cycles Synchronize if They are in Close Proximity for Large Amounts of Time

menstrual-cycleDo women’s menstrual cycles sync up over time if they are in close proximity to each other? Being someone who lives with 4 girls and 1 other boy, this knowledge would be very beneficial. I feel like I should know if the estrogen ocean I’m swimming in will have very stormy seas every 28 days. The belief that this occurs is widely known. In 1999, one study reported that 80% of women believe this phenomenon occurs and 70% of them find it pleasant to have this be the case. The truth, like so many other things in science, is controversial. The main body of evidence, however, suggests this is nothing but a myth and it’s really just random chance when this synchronization is occurring. Let’s find out why.

A woman’s menstrual cycle is counted from the first day she begins menstrual bleeding until the first day of the next menstrual bleeding. This cycle can vary from 21-35 days, although the average is 28. It’s separated into 4 different phases- the menses, follicular, ovulation, and luteal phases.

The menses phase is when the woman is actually bleeding. The average time frame of this bleeding is 3-5 days, though 2-7 is considered normal. It’s this phase people are paying attention too when they talk about menstrual cycles synching up. They look at the first day of bleeding as the start point. The follicular phase is where the follicles in the ovaries begin to mature and form an egg. Simultaneously estrogen levels rise causing the lining of the uterus to thicken. Ovulation then occurs which releases the egg due to an increase in a hormone called the luteinizing hormone. The final phase is the luteal phase. The egg is released from the ovary and travels down the fallopian tubes and in to the uterus. If the egg is fertilized and attaches to the uterine wall, pregnancy begins. If not, estrogen and progesterone levels fall causing the thickened lining in the uterus to shed and menses begins again.

The entire process from the start of menses to the beginning of the next menses is mediated by hormones. Because of the hormone mediation, proponents of the synchronization theory claim that pheromones can affect hormone levels and thus, affect the cycle.

The theory began in 1971 when a psychology student at Harvard (Martha McClintock) published a paper in “Nature” stating that women who spend a lot of time together will tend to have their menstrual cycles synchronize over time. She theorized that pheromones affected the hormones of the women causing the synchronization. Pheromones affecting people in general is an extremely controversial scientific topic in and of itself. It would take an entire article to do it justice so I won’t be going into it here. That being said, in 1998, McClintock published another paper in “Nature” which found that if a woman was exposed to cotton pads soaked in the sweat of women in their follicular and luteal phases, the phases of their cycles were significantly altered.

The controversy lies in how you interpret the data from McClintock’s, and others, studies. The differing lengths of cycle times combined with a myriad of other things that affect cycles, causes the results to be simply a matter of chance. One researcher, H Clyde Wilson, outlined three serious errors involved in McClintock’s research. When you correct for these errors, the evidence for synchronization evaporates.

So why is this so complicated?  Defining what exactly synchronization in cycles is can be extremely difficult. While there are averages, most women tend to have their own unique cycle that can be affected by many things. Suppose you have one woman who has a 28 day cycle and one that has a 29 day cycle. If they both start menses at the same time this month, they will be off by one day on the following cycle and then 2 days on the one after that. Over time they will, once again, have a cycle that will coincide with one another. To take it one step further, let’s say you have women at both ends of the “normal” range for cycles. The first has a 21 day cycle and the other has a 35 day cycle. If the women who has a 35 day cycle starts on January 1st, and the women with a 21 day cycle starts on January 15th, both of their cycles will start approximately at the same time the next month (February 4 and February 5 respectively). Those two women could say “Wow, we were two weeks off last month and this month we have been spending a lot of time together and look, our periods are synchronized!”

Other things to consider when talking about cycle periods is that each individual has different patterns of normal. Some have regular patterns, some have variable patters, and others have regularly irregular patterns!  Those same women can also experience different cycle lengths and regularities depending on several environmental factors such as strenuous exercise, puberty and menopause, low body fat, and extreme weight loss or gain have all been shown to affect cycle lengths.  Due to the fact that hormones regulate the entire process, anything affecting hormone levels, will in turn, affect menstrual cycles.

Simply interpreting the data and not being cognizant to our “confirmation bias” (the tendency to search for or interpret information in a way that confirms one’s preconceptions) can lead to errors in statistical analysis.

Take for a specific example two subjects involved in a study that started on July 1st. Subject A with a 28 day cycle starts her menses on July 25th and then again on August 22nd.  Subject B, with a 30 day cycle, has her onset start on July 5th and then again on August 4th. If you believed in the “synchronization” theory and were not aware of your confirmation bias, you could look at these numbers and conclude their cycles were 20 days apart (July 25th and July 5th) in the beginning and are now 18 days apart at the second onset (August 4 and August 22). Thus the subjects are probably synchronizing. The truth is, the opposite is actually true, they are diverging not converging. Subject A would have had menses start on June 27th as well. Not looked at by this study starting on July 1st. That puts both subjects start as 8 days apart in the beginning (June 27 and July 5). The second cycle being July 25th and August 4th makes them 10 days apart and diverging. The exact opposite of the original conclusion.  So you can see from this how it would be easy for a study on this subject, if not performed correctly, to provide very wrong conclusions even without all the noise of things like regularly irregular periods and other factors effecting hormone levels.

The idea that synchronization is simply a matter of chance has been replicated by several other studies since McClintock’s “groundbreaking” conclusion in 1971. One of the most famous was done by Zhengwei Yang, of North Sichuan Medical College in China, and Jeffrey Schank, of UC Davis in 2006. They found women living in groups did not synchronize their cycles. They looked at and analyzed previous studies claiming synchronization occurs. After correcting for statistical errors, they showed those results were really at the level of chance. They went on to show that because of cycle variability, they produce convergences and divergences in menses onsets and this explains the perception of synchrony.

In the end, Michael Shermer in the book Don’t believe everything you think, said it best, “Smart people believe weird things because they are skilled at defending beliefs they arrived at for non-smart reasons.”

Whether you believe McClintock (the biggest proponents of synchronization out there, starting back from her days as a student) and say that your menses converging and diverging is a result of pheromones affecting your hormones, or you take a step back and look at the main body of research that gives a non-pheromone mediated reason for why the convergences and divergences happen, the result is the same. Due to the differences in female cycles, and the environmental factors that can affect them, there will inevitably be certain times when the women in your family will all be in their menses phase at the same time. I shudder to think! Father-son vacation anyone!

If you liked this article, you might also like:

Bonus Facts:

  • The average age a girl starts having menstrual periods is between 11 and 14. Beginning around the age of 39-51 a women will begin to have fewer menses. Both age ranges will tend to have cycles that change consistently or are longer. The teenager will experience her menses evening out and become consistent while the older women will have hers get longer and then stop. Either way, if I haven’t said it already, I’m glad I was born a man! You ladies get the short end of the stick when it comes to periods at least. :-)
  • The first menstrual period of a girl is called menarche. The average age of menarche dropped from 12.75 to 12.54 years old from 1988-1994. This mirrored the results of other research that found the average age of menarche dropped 2 ½ months between 1963-1970. The authors of that research published their findings in the journal Pediatrics in 2003. They concluded that the decrease in menarche paralleled the increase in the body mass index of the US population, suggesting that higher relative weight was strongly associated with early onset of menarche.
  • In case you didn’t already know, the most common cause of a missed period is pregnancy.  Shocker!
  • As stated in the main body of this article, hormones mediate menstrual cycles. Progesterone and estrogen levels are one example of how hormone levels affect menses. Progesterone is secreted by empty egg follicles after ovulation. This causes the endometrium (the lining of the uterus) to secrete proteins to prepare it for fertilized egg implantation. It also helps in the growth and maintenance of the endometrium. Teens and women close to menopause have changing levels of progesterone and estrogen. The result is heavy menstrual bleeding and an increase in cycle length.
[Image via Shutterstock] Expand for References
Share the Knowledge! facebooktwittergoogle_plusredditpinteresttumblrmailfacebooktwittergoogle_plusredditpinteresttumblrmail
Print Friendly
Enjoy this article? If so, get our FREE wildly popular Daily Knowledge and Weekly Wrap newsletters:

Subscribe Me To:  | 


  • I was on a college summer leadership project for 7 weeks. We stayed in the same building (an old 1920’s hotel, girls on one floor, guys on the other – many to a room) worked and played together.
    Every week the guys & girls would have their own meeting times together.
    One of the girls told me about the last meeting in which the girls took a poll. Their periods were all occurring within a 3 day window.
    This was about 20 girls.

  • Ok I dont want to sound creepy or weird. I will just tell you about my experience.
    I live with an adult woman and two female cats.
    I swear to god, before the cats were spayed, their monthly time when they got into heat always fell within the same time the female human in the house had her period.

  • When I first started college one of my floor mates in my dorm told me that by the end of the year our periods would be completely synced up, and of course I was skeptical. It seemed highly unscientific and more like an urban myth. This article makes sense, because there were times when it seemed I was synced up with some girls. But it was really that throughout the month there was always someone on her period, so of course when my time came I was synced up with someone.

  • The synching of women’s menstrual cycles is attributed(anecdotally) as the reason for birth of the music style ragtime. In the days western expansion, as settlements became towns and then cities, services followed. Houses of prostitution provided on service. The synching of the ladies menstrual cycles meant that each month the house was effectively closed with nothing but the bar to bring in money. The piano player was relied upon to entertain the customers and thus the style became known as “ragtime”. Not scientific, but certainly a satisfying explanation.

  • Charlene A. Howell

    How is it that a man is allowed, with little question, to speak on this female issue that dates back to the days of creation? I’ve had my cycle for 20 yrs now, and have noticed that when I spend more than 3 weeks in close proximity of another woman or women and my cycle hasn’t already begun, it’s almost a guaranteed certainty that our cycles and premenstrual symptoms will synchronize. Weird but true…

    • Daven Hiskey

      @Charlene Howell: Because the man is a medical professional maybe? Or, let me put it this way, I’ve taken a crap literally thousands of times in my life, but that doesn’t make me an expert on the sphincter or the digestive tract. ;-)

    • Agree with you 100% (and wth are those panty liners in the pic good for? LOL NOT anyone’s period, that’s for sure! :D

    • How ignorant. What if I said a woman had no place sounding in on male health issues even if she’s a doctor?


  • 2 words: boot camp. Cycle “sync up” is irrefutable FACT!

  • I have 2 daughters, my period is always faithful and always came around the 25th so holidays :( were no fun lol . Once my girls started my date kept changing and theirs too until now we all 3 start at the same time or within a day of one another ! When all of this started happening I was scared that something was wrong with me but that was all it was…we were syncing so I am 100% , without a shadow of doubt positive that this is real !!!! Now feel sorry for the 2 boys ( my husband and my son) ha !

  • Let me add that when I was a teen my step sis and I lived together off and on and EVERY time we started living together we would sync back up ! My Parents made it a running joke so believe what u will …it’s real !

  • why jesus fuck burger king. must you write this long text for you publisher, why can’t you just answer the question. YES OR NO?

    • Daven Hiskey

      @Annoyed Penguin: The interesting part is why “yes or no”. Also, had he just said that, what reason would you have to believe him?

  • Period synchronization is a fact! I am a female who seems to sync up with anyone who i spend quite a bit of time with. For example i lived with my mother and i noticed that we started within two days of each other. My cousin moved in and the next month my period (along with my mother) started the same exact day as hers. I gained a best friend in that time and noticed that within a month or two my period moved and started in the same week as hers. Then when i moved into a dorm my period did a complete 360. It MOVED from the end of the month to the middle. Which happened to be the same week as my roommate. I still have the same roommate (its been two years) and it has not changed. So the fact that you are writing about something you dont even experience, nor understand stuns me. Your just going off of what MALE scientist are saying. I feel i should tell you that science/studies cant explain everything.

  • Well it’s pretty common here. I have 4 sisters and a mom and I as well who are synced within the week. Actually me and my mom the same day every month. Doesn’t seem like a myth to me considering it happens every month. Every now and then I’ll stay with a friend who’s female and we’ll be synced within that week.

  • He’s not answering “yes” or “no,” and I don’t believe he was planning on giving a definitive answer. Instead, the author is saying that you can’t just believe a scientific study off the bat. Since menstrual synchrony is difficult to define, it’s going to be difficult to prove or disprove. He’s telling his readers to look for the errors and biases within a study and then to draw conclusions for ourselves. With this particular topic, you need to look at the sex of the researchers, a definition (if one is provided), as well as how the subjects were chosen and how the data was collected.

    Personally, I have experienced menstrual synchrony, thus I completely believe in its existence.

  • Bill Guy the science nye

    ITT: girls who don’t understand anecdotal evidence argue with scientists.

  • Bill go back to 4chan

    “Smart people believe weird things because they are skilled at defending beliefs they arrived at for non-smart reasons.”

    Women flood the comment section with examples of their periods synchronizing, and we say they’re wrong because we can point out some flaws in a scientific article from the 1970’s? That doesn’t seem so smart.

    If there was a preponderance of scientific evidence cited in this article I’d feel it was more credible. As it stands, I think it’s smarter to trust the women who have lived the “science.”

    • Bill’s point is that anecdotal evidence is one of the lowest levels of evidence in existence. This article is mainly written to say that there is no strong evidence and therefore we can’t assume to know.

  • As a retired teacher I can assure you that synchronization is a fact. You can ask any janitor.
    When school starts all girls have independent cycles and the sanitary napkin dispensers handle the load but by Halloween they are full 3 weeks and empty the fourth each month. Teachers have to keep a box of napkins for girls in her classes when the dispensers are empty.

Leave a Reply

Your email address will not be published. Required fields are marked *