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Why Do Women's Menstrual Cycles Get in Sync?

Imagine this, a man's worst nightmare (besides the abolition of football): All the women in his life are on their cycles. At the same time. Sound like science fiction? Try science fact. Women who spend at least three days a week in close contact with other women eventually start to see their menstrual cycles come into sync with each other (provided they are all healthy and not pregnant). Why is this? The answer deals with humans as a species and our need to reproduce effectively, and also with how our bodies are designed to work to accomplish this.

Humans are a social species, like many others. We gather in groups for protection, for companionship, and for survival. More members in the group means more food can be gathered, more tasks can be accomplished. Living in groups also gave early humans a bigger gene pool for reproduction. The problem with human reproduction is, how do you ensure as many babies are being born as possible to ensure survival as a species?

Hormones. We all secrete hormones through our skin. They convey many different, important messages, though many people have lost the ability to consciously receive and interpret them. Hormones can tell us if someone else is healthy or sick, male or female, happy or unhappy. Someone else's hormones can also affect your body and your own hormone production. A recent study found that women secrete hormones during their ovulatory cycle (fertile window) that make men view them as being more attractive and more open to sex. Another study has found that women can tell if a man is aroused simply by his sweat. Hormones in the man's sweat increase if he is aroused, and women subconsciously pick up on these "clues".

But how do hormones make women's cycles sync together? As said before, hormones from someone else can affect your own hormone production. Biologically, the way to ensure maximum reproduction is to have maximum opportunities for reproduction. Early human groups resembled harems, with several women to a few men. This meant there were many women to get pregnant, but not many men to impregnate them. Nature's answer to this problem was simple: Women would produce hormones during their fertile period that could jumpstart other women's bodies into being fertile as well. Of course this takes time, which is why today it takes a few months for women to "get in sync". But back then women spent more time together than today, so their bodies were in sync before they even reached puberty. Women being fertile together gives the males a better chance at producing more offspring, since women are only fertile for a few days each month, and can only have one baby a year, whereas males are fertile all the time, and can produce as many offspring as there are available, healthy females.

Conclusion: Women get their periods together not to terrorize men, but as a biological response to hormones and the human instinct to reproduce (though some would say we have accomplished a little TOO well).

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Comments (34)

These are interesting facts I never thought about before...thanks for sharing.

terrorizing men is just a bonus! (trust me, its worse for us women, than it is for the men)

I always wondered about this... fascinating!

Lauren

Early human groups did not resemble harems. Archeological digs have found a fairly even male to female ratio. It was only a few men, usually the more respected or powerful men in the tribe, who had enough resources to support multiple wives and all their children. Also, how would it help to produce hormones that jumpstart other women's bodies into being fertile at the same time, if everyone is still only fertile about three days out of the month?

This is a theory I've heard before, but I've never really understood it.

john

That is some crazy stuff.

john

That is some crazy stuff.

Ellen

Okay, so if women have a tendency to "sync cycles", then is it possible for a woman who has went through menopause to affect a woman still having cycles? 8 months ago, I began taking care of my great grandmother & about 4 months into it, I quit having cycles. So far, my doctor can't find any medical reason for this & I have been regular sense the first one I ever had. Any imput??

Kelly M.

Ellen I think I'm expreiencing the same thing except that my best friend is with child and I think my cylce is in sync with hers. Because her and me would pretty much hang out all the time and ever since she has been pregnant the last 8-10 weeks I haven't had a cylce since my october one. I know I'm not pregnant but its strange to me because I too have never missed a period before. I would like to know if this has happen to other females too.

Kelly

I agree with Tory.

erin

this is very true- my cycle is 35 days. every year that i do my seasonal job from sept-jan, i go to 22 days. i work with lots of women and i hate that this happens!

Maria

I'm only 13 so I started my period like last year and when I first started it wasn't regular yet. But then my mom was like our periods are starting to sync and I was like what? Now I know.... Cool

bob

This theory is pretty much crap. Why would everyone ovulating at the same time ever help a man get more women pregnant? At the very best it would do nothing statistically but more than likely it would actually cause him to impregnate less of them than he would if they weren't synced. Men are the most fertile when they have sex less than 3 times a week. The more sex they have beyond that the worse the sperm are. So how would it make sense that a man living in a "harem" would need to have sex with all the women at once? Furthermore, humans pretty much universally form monogamous sexual relationships with exceptions being few and far between meaning the "harem" part of this theory was completely just made up.

The most promising theory I've ever heard for why women who are close to each other tend to have similar cycles is the shared stress idea. Basically there are many stresses that can effect ovulation (i.e. illness, work/school related problems, social issues, daily exercise or lack there of) and because women who share close quarters often have the same stresses this can, over time, cause their cycles to even out. Also, I think society uses a lot of selective attention with this issue and it is not as prevalent at people like to think.

PGM

It's hard to trust anything a science writer writes if they can't get the underlying factual information correct. Animals don't secrete hormones to their environment for communication between individuals. Hormones are internal signaling molecules that influence cells and tissues within the individual. Chemicals released to communicate with other individuals are called pheromones, and are a different set of molecules than hormones. The phenomenon of synchronized menstrual cycles is pretty well established. Hypotheses about how or why it might be beneficial require some firm information about the social environment in which this phenomenon first evolved. Since we don't know that with any degree of certainty, these evolutionary psychology speculations about why humans behave and respond in the ways they do will always be fairly speculative.

dave

nah, thats dumb... if men were fewer in number and woman all became available for one 2-3 day window... there are a limited number who will be able to get pregnant as a dude can only spuff so often over 4 days...

staggered cycles means dudes can get pume pume all months round, impregnate all month round...

sounds like a better way to go about things

Skeptical

What are your credentials? Are you a doctor or a scientist? This sounds like you just googled a bunch of crap to write an article.

Skeptical

What are your credentials? Are you a doctor or a scientist? This sounds like you just googled a bunch of crap to write an article.

scamoosh

this doesn't make sense at all, and it's wrong on two fundamental levels.

1. It assumes group selection theory is correct

2. Even if group selection theory was correct, why would genetics restrict the men to a window of a few small days to get ALL the women pregnant?

If such a phenomenon exists, it is much more likely to do with genetic competition between women than this ridiculously flawed theory

scamoosh

this doesn't make sense at all, and it's wrong on two fundamental levels.

1. It assumes group selection theory is correct

2. Even if group selection theory was correct, why would genetics restrict the men to a window of a few small days to get ALL the women pregnant?

If such a phenomenon exists, it is much more likely to do with genetic competition between women than this ridiculously flawed theory

Not correct

Its true that women's periods do sync together as I've experienced it many times. However I don't think this is to increase pregnancy chance.

I think it is more likely to protect them from pregnancy.

Women naturally conceal their ovulation so men have to stay with them all the time, hence why we mainly have monogamous relations.

With an entire group of women having their periods/ovulation at the same time it is less likely for the man to impregnate more than one women, again encouraging monogamy.

Not correct

I believe that women do sync their periods but not for the reasons stated here.

Women naturally conceal their ovulation which means the men have to become companions with them to ensure pregnancy,hence the reason we mostly have monogamous relationships.

If a group of women all ovulate together it would be even less likely for the man to get more than one women pregnant which again encourages monogamy which is beneficial to women with children.

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Sue

My daughter worked the summer season overseas but as soon as she came home she would have an affect on my cycle... she is now very much pregnant and my breasts have grown !! I am sure this is to do with hormones... has anyone else had similar experience??

rory

I clicked on this article hoping to read up on this interesting phenomenon. However, I could not bring myself to read any further than the first sentence: "Imagine this, a man’s worst nightmare (besides the abolition of football): All the women in his life are on their cycles."

That's right, because we all know how awful menstruation is for MEN. MEN suffer so much from WOMEN'S periods. Because, you know, women aren't even HUMAN when they're on their periods. They magically morph into something demonic and evil and terrifying and--worst of all--SOMETHING THAT DOES NOT PLEASE MEN! What could be worse than MEN being inconvenienced? As if MEN didn't go through enough awful things already! The horror!

I am thoroughly disgusted to find this misogynistic bullshit on a website that is supposed to be HELPFUL. Rest assured I will not be using this website in the future, and will not recommend it to anyone else.

rory

I clicked on this article hoping to read up on this interesting phenomenon. However, I could not bring myself to read any further than the first sentence: "Imagine this, a man’s worst nightmare (besides the abolition of football): All the women in his life are on their cycles."

That's right, because we all know how awful menstruation is for MEN. MEN suffer so much from WOMEN'S periods. Because, you know, women aren't even HUMAN when they're on their periods. They magically morph into something demonic and evil and terrifying and--worst of all--SOMETHING THAT DOES NOT PLEASE MEN! What could be worse than MEN being inconvenienced? As if MEN didn't go through enough awful things already! The horror!

I am thoroughly disgusted to find this misogynistic bullshit on a website that is supposed to be HELPFUL. Rest assured I will not be using this website in the future, and will not recommend it to anyone else.

doesnt matter

Rory, you're an idiot. It's being sarcastic. Not all woman on their periods are bitches. Maybe you annoy them? Or you deserve them nagging at your dumbass because you forgot to call them to set up a date.

It sounds like you have more PMS than women. Sorry to break it to you,bud. ;)

Crash2Parties

Rory, I thought the same thing as I read that first sentence. & to be honest the article itself didn't provide that much useful info (mentioning pheromones would've been a little helpful).

Personally, I've wondered if synchronizing was a combination of controlling pregnancy *and* a manifestation of female primate hierarchy politics. I've noticed that there seems to be a pattern of who syncs of off whom (ie who leads and who follows, if you really need to use that model)...but it doesn't quite fit what you'd expect given modern social structures.

Crash2Parties

Rory, I thought the same thing as I read that first sentence. & to be honest the article itself didn't provide that much useful info (mentioning pheromones would've been a little helpful).

Personally, I've wondered if synchronizing was a combination of controlling pregnancy *and* a manifestation of female primate hierarchy politics. I've noticed that there seems to be a pattern of who syncs of off whom (ie who leads and who follows, if you really need to use that model)...but it doesn't quite fit what you'd expect given modern social structures.

good one, thanks

KT

hahaha ahh to doesn't matter, speaking to rory..i am just about completely certain that rory is a girl! and that's why SHE was mad, im sorry but that is just hilarious that you would think its a guy...wow, as if a man would rant about that and why would a guy look that up in the first place.

Bobbie

It's funny, I'm 45 and everybody has ALWAYS synced to my cycle. I have had a lot of roommates in my life and the first month I'm around them, they will actually have 2 periods (their normal one, and another one when I get mine), and remain on my schedule until I'm out of the picture. A friend of mine has come to stay with me while she goes back to school. She was going through menopause for the past 6 months. Within a couple of weeks she was buying supplies and cursing my name! She has been as "regular" as me for 3 months now. I thought that the advantage of syncing together (way back when) had more to do with the survival of the babies. Mothers often died during childbirth back then and if there were other lactating women around, then an orphaned infant would have a greater chance of survival.

Modern women have quite a lot of problems with their menstrual cycle because of a lot of stresses!

perfectresume.org

I guess that means the guys in my circle of friends will be absolutely miserable once a month :) But hey, at least it's all at once so they get a break...

LIGHTBULB!!!!!! Guys go crazy if they're surrounded by hormones, so maybe it was partly made to preserve their sanity and give them a break!!! Yeah, I'm stupid. XOXO Claire

this doesn't make sense because even if it was true then it couldn't possibly be true. I have female friends I hang out with, and I have some that don't hang out with each other. And if they both started syncing to the time I have my period, that so must their friends, and then my friends girlfriends must sync to thiers too, and their friends sync to their cycles and so on. And eventually just every women would have their periods at roughly the same time. and not all women do have their periods at the same time, that alone should show it's a myth. and statistics to prove this would be difficult to come by, considering people would tend to notice the friends they are "syncronized" with more so than the ones they aren't. It could be coincidence. a period is like on average, 5 days long, almost a week for some women. and there's about four weeks in a month, so roughly a .20 percent chance or more that they would "sync" just by chance alone.

Hi and thanks for the 411 this was great for my young daughter to gain more innerstanding of her body and how it affects other women around her.

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