Unraveling the Myth of Menstrual Synchronization

What the research reveals

Henrik Sorensen, Getty Images

Many people have heard the idea that if there are two or more people who menstruate and spend enough time together in relatively close confines, their menstrual cycles will eventually “sync up” and run on the same schedule. Basically, one menstruator drags another into her orbit, achieving menstrual synchrony. Often, there are stories of how this has happened with sisters, friends, spouses, office mates, or dorm mates. 

This myth comes around over and over again, and some people get quite, um, perturbed when they are corrected. On the surface, it’s a seemingly harmless myth, but the mythology of menstrual synchrony is related to the harmful idea that menstruation is toxic, and one proof offered was menstrual miasma, an unseen and dangerous force. There are historical claims of menstruating women who were essentially lesser supervillains with the ability to spoil milk, ruin mirrors, and wilt flowers. As much of our history was written by men, a lot of this menstrual mythology is not borne from harvest goddess empowerment but rather the idea that many women were witches who sought to control and pollute men. And lest you think this idea of menstruating women as the great polluters is an ancient one, there were two letters published in 1974 in The Lancet, a leading medical journal, suggesting that menstruating women could wilt flowers. Yes, 1974! And it wasn’t the 1974 April Fools’ edition either. 

Yikes!

A Flawed Study

I’m sure the idea of menstrual synchronization has been around as long as the patriarchy. But narrowing our search down to what science has to say leads us to the study that brought the concept of menstrual synchronicity (a great name for an album, by the way; The Police were just missing one word) into the scientific mainstream. It was published in the journal Nature in 1971 and was written by Dr. Martha McClintock, a psychologist. The gist is that 135 women aged 17-22 living in dorms were surveyed and grouped based on who they lived with or spent time with. At the end of every two months, they were asked about their last two periods, and the study took place over a six-month time frame. These women were not recording the first date of their last period in real-time rather, they provided this from memory.

There are some major issues with the study. For starters, women taking oral contraceptives were inexplicably included. And then, there is the accuracy of menstrual cycle recall. In one study where women prospectively recorded their menstrual data and then were later asked about it (so they also provided retrospective data), 54% got the date of their last menstrual period exactly correct, and 85% were correct within 3 days of the start of their period. As expected, the further out from the last period, the more likely there was an error recalling the date. Meaning a study that depends on an accurate date for the start of menstruation can’t rely on asking women when their last period started and, even more importantly, their second to last period.  

As an aside, not remembering an exact date two months ago is not a female or a menstrual phenomenon. I know I had a power bar for breakfast one morning last week, but I’m not 100% sure which day because, at the time, it seemed unimportant. 

There is even a menstrual recall phenomenon that could make it seem as if women are synchronizing up. There is a study that shows when women are asked to recall the date of their last period, they are statistically more likely to pick the dates 1, 5, 10, 15, 20, and 25, so if women are statistically more likely to pick one day over another that is a major confounder in research that is trying to determine if periods started on the same day or not. This is also why retrospective data looking at the impact of vaccines on the menstrual cycle isn’t very useful.

In addition, and perhaps most importantly, multiple studies by different researchers have looked at menstrual synchrony and found that it does not occur. I mean multiple studies. There is actually a lot of good data here (for once!). The menstrual app Clue also analyzed their data and found that this phenomenon doesn’t exist. 

Finally, no well-done study actually proves the phenomenon of menstrual cycle synchrony. 

There is No Valid Hypothesis

The 1971 paper that made the bold claim about menstrual synchrony suggests that this phenomenon is seen in mice and that it is related to pheromones. A pheromone released by the ovary was one of the contenders. What that unknown chemical actually did was, well, unknown.

However, women have a menstrual cycle, and mice have estrus, so comparing us to mice is not an ideal model. Also, a mouse can have 5-10 litters a year, and humans, well, you get the point. And finally, most experts don’t believe humans have pheromones. Despite what the perfume industry tells you, there is no definitive proof, and humans don’t have a functioning vomeronasal organ, which is the apparatus needed to detect pheromones (in fairness, this latter piece of information is something that was probably not known in 1971). 

The system that governs the menstrual cycle, meaning the hormone signals from the brain and how the follicles develop in the ovaries, is quite robust. It takes a major calorie deficit or stressor to impact the cycle, and when this happens, it is typically a lengthening or a delay in the next period. For example, this is the effect we see in the small percentage of people who have a menstrual cycle change with the COVID-19 vaccine: a small delay in the onset of the next period. However, the cycle quickly goes right back to baseline.

Many people wave their hands and say “pheromones” in support of menstrual synchrony, but an actual valid hypothesis of how these “pheromones” work has never been realized in the slightest. It is really not a valid scientific argument to wave hands and say, “pheromones.” For person A to control person B’s cycle, person A must have a chemical that can travel through the air, be absorbed or detected in some way by person B, and work continually throughout the menstrual cycle on the brain, the follicles in the ovaries, or likely both.

We can do some cycle control with hormones. For example, I can put the follicles in the ovaries on ice (so to speak) with birth control pills and then trigger them to start developing with ovarian stimulation medication. Or, if I give someone a progestin for several days, I can usually delay the onset of their next period. However, the effects of these medications I’ve just described are limited to the cycle they are used. Also, biologically I can describe exactly how they work. But for menstrual synchronization to work, we need to believe that something like this actually happens with an undiscovered chemical that travels through the air that not only affects the timing of one cycle but, given menstrual cycles can vary in length of up to 7 days, this unknown chemical messenger must be able to work throughout the entire menstrual cycle, cycle after cycle, to keep two people in tandem. It’s just not biologically plausible.

There are several reasons why people might have the illusion of menstrual cycle synchrony. The first we’ve discussed already, an inaccurate recall of the exact first day of their last period. And then there’s the math, If you take two people who menstruate, chances are they do not have exactly the same cycle duration. So if they start off out of sync, mathematically they will eventually have two or three cycles, or more, within one year that start in close proximity and then move out of sync again. And finally, we all have recall bias, which is essentially selective memory (this doesn’t have to be intentional). People likely only remember the cycles they thought they were in sync, so they think it’s happening more than it is. 

It’s possible that family members, a mother and daughter or two sisters, could have similar cycles based on a combination of genetics and shared environmental factors, but that’s not cycle synchrony; it’s having similar biology and a similar environment.  

What’s the Harm in Believing in Cycle Synchrony?

Magical thinking about the body seems limited to the female body, which is concerning. It's a holdover from when it wasn’t uncommon to believe that women could cast hexes and spells, and the uterus was thought to be a wild animal within a wild animal. And we deserve better than magic.

Multiple studies have shown menstrual synchrony is not a medical phenomenon. This is a case of investigators taking a claim seriously and researching it. For too long, women have been forced to accept Old Wives’ Tales about their bodies and how they work, while men got the funding for science, so to turn away when we have good science is…disheartening. 

I know it’s hard to shed beliefs, especially something that you believe you have seen with your own eyes. The fact that people believe menstrual synchrony to be true is likely because they hear about it a lot, and we all mistake repetition for accuracy. In addition, many claims, even wild ones, lead people to falsely believe an idea is valid. For example, lots of people claimed to have seen the Loch Ness monster, so there must be some truth to it! But multiple anecdotes are simply that. There are also multiple anecdotes of people who say they have never experienced menstrual cycle synchrony, but those never go viral.

But we don’t need anecdotes because it’s 2023 and no credible research proves the existence of pheromones, and there isn’t even a semi-realized biological hypothesis for menstrual cycle synchrony. However, there are many excellent explanations for how cycles could appear to be synchronized, and, most importantly, a lot of excellent research proves cycles don’t think.

And we finally have a case of lived experiences being researched, and now that we have multiple studies and scientific explanations, we need to move beyond the mythology because we deserve better.

And if you want more facts like this, I have an entire book on the subject coming out on January 23, Blood: The Science, Medicine, and Mythology of Menstruation, and I am excited to say it received a starred review from Publishers Weekly (which is kind of a big deal!). If you are interested in pre-ordering, this is the link. And if you want an autographed copy and might not catch me on book tour, you can pre-order one (United States only) from Book Passage (and I am more than happy to personalize with any special requests, “Fuck the Patriarchy,” always seems to be a favorite). 

References

Mclintock M. Menstrual Synchrony and Suppression. Nature. 1971;29:244-245.

van Oppenraaij R, Eilers P, Willemsen SP, van Dunné FM, Exalto N, Steegers E. Determinants of number-specific recall error of last menstrual period: a retrospective cohort study. BJOG. 2015 May;122(6):835-841.

Wegienka G, Baird DD. A comparison of recalled date of last menstrual period with prospectively recorded dates. J Womens Health (Larchmt). 2005 Apr;14(3):248-52

Ziomkiewicz A. Menstrual synchrony: Fact or artifact? Hum Nat. 2006;17:419-32. 

Schank JC. Do Human Menstrual-Cycle Pheromones Exist? Human Nature 2006;17: 448-470.

Ziomkiewiz A. Menstrual Synchrony: Fact or Artifact? Human Nature 2006;17:419-432.

Wilson HC. A critical review of menstrual synchrony research. Psychoneuroendocrinology. 1992;17:565-91.

Trevathan W R, Burleson MH, Gregory WL. No Evidence for Menstrual Synchrony in Lesbian Couples. Psychoneuroendocrinology. 1993; 18:425-435.

Weller A, Weller L. Examination of Menstrual Synchrony among Women Basketball Players. Psychoneuroendocrinology 1995;20:613-622.

Weller A, Weller L. Prolonged and Very Intensive Contact May Not Be Conducive to Menstrual Synchrony. Psychoneuroendocrinotogy. 1998;23:19-32.

Strassmann BI. The Biology of Menstruation in Homo sapiens: Total Lifetime Menses, Fecundity, and Nonsynchrony in a Natural-Fertility Population. Current Anthropology 38:123-129.

Strassmann BI. Menstrual Synchrony: Cause for Doubt. Human Reproduction 1999;14:579-580

Clue, blog post, accessed 10/28/2023 https://helloclue.com/articles/cycle-a-z/do-menstrual-cycles-sync-unlikely-finds-clue-data#:~:text=Unlikely%2C%20finds%20Clue%20data,-%E2%80%94%20March%208%2C%202017&text=Menstrual%20health%20has%20long%20been,up%2C%20or%20%22syncing.%22