Do Periods Really Sync Up Among Friends?

You’ve probably all heard about how close friends’ menstrual cycles sync up when they live together, but is there any truth to it or is it simply an urban legend? And can the synchronicity occur with people you only interact with online? There’s plenty of anecdotal data, but let’s see what science has to say about it.

Studies on menstrual synchrony have had mixed results. The first study of the phenomenon was published in 1971; after psychologist Martha McClintock noticed that many close friends in her all-girls dorm had their periods at the same time, she polled 135 dorm dwellers about when their periods started at three different times during the school year. Her study found that in October close friends started their periods 6.4 days apart on average, while in April, after they’d been living together longer, the difference was down to 4.6 days. However, a 1992 re-examination of her data by H. Clyde Wilson of the University of Missouri”“Columbia found that the initial data had been misinterpreted and the difference thus became statistically insignificant. Other studies have been inconsistent, with one grouping apparently demonstrating synchrony and another not, with no discernible explanation as to why.

Part of the problem may be with how the studies are performed. Simply looking at a snapshop of period start dates doesn’t necessarily tell you much about whether their cycles were affected, or simply following their usual but different patterns. As an example, let’s look at four hypothetical students as seen in the chart below. When they move in together at the start of the school year, A & C have 28 day cycles and B & D have 31 day cycles, with B & C starting their first cycle of the school year on August 15 and A & D starting on September 1. If you simply take a snapshot of their cycles in January, students A & B appear to have synchronized, as have C & D. However, D has actually gone through one cycle fewer than C. By May, the apparent synchronous pairs are once again A & D and B & C, though in both pairs one student has had more cycles than the other. All of the students have retained their usual patterns, but picking and choosing what data you look at can give you the impression that there’s something else at work.

chart of 4 students with 28 or 31 day cycles starting on August 15 or September 1
Hypothetical period distribution over the course of one school year

There have been several studies that show that ovulation can be affected by exposure to female pheromones. In a separate study from the one mentioned above, McClintock was able to alter some women’s cycles by exposing them to underarm secretions from other women who were about to ovulate. Other studies seem to point to similar results with the same secretions or with exposure to secretions from breastfeeding women. While this is interesting and merits further study, researchers still don’t know why some women respond and not others, and why women who do seem to synchronize don’t stay in synchronous cycles over a long stretch of time. Also, syncing ovulation doesn’t necessarily cause periods to sync up; due to varying menstrual cycle lengths, it can actually push period start dates farther apart.

While it hasn’t been specifically studied, I can say with some certainty that periods absolutely cannot sync up over the internet. I know, it’s fun to joke about it and it’s therapeutic to know others are going through PMS at the same time as you, but it’s mostly due to coincidence, sample size, and observational bias. If you complain about cramps on tumblr and five friends chime in that they’re about to start their period too and several other say they just started, it can seem like there’s a mechanism at work affecting all of you. But if you complain the next month, some of the other PMSers may have already started, some who had already started last month may just be PMSing, some may not be having their period at all, and others may chime in who were unaffected last month. If you have, say, 180 followers and (based on some highly unscientific polling of my friends) about 2/3 have a monthly period and many others have periods every three months or so due to their birth control, on any given day of the month about five of them should be starting their cycle, and a few dozen more will have already started or will with a few days. That’s not syncing, it’s math. You don’t notice the other 100 or so who aren’t menstruating, so it seems like a bigger deal. This applies to living in dorms as well. When you have several hundred women living together, of course some are going to start their periods at the same time. You notice it more amongst your friends because you’re more likely to talk to them about it than someone you aren’t close with. Sorry!

Scientific American: Do Women Who Live Together Menstruate Together? by Anna Gosline
Wikipedia: Menstrual Synchrony

By [E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

24 replies on “Do Periods Really Sync Up Among Friends?”

huh. when I was growing up, my mom, sister, and I would all sync up, so I always thought the idea had some weight to it. But everyone and their situations are all different… kind of nice that it’s not a hard-and-fast rule, or else those who didn’t stock up on supplies would be running to a sold-out Walgreens… yuck! lol

I’m on birth control, so mine doesn’t change.  But my roommates’ cycle has always been sporadic–usually every other month– and since we moved in together, she’s synched up with mine completely.  She always starts 2 days after me.  It’s really weird….

So interesting! In the dorms, and at summer camp, and at work (at times in my life, I’ve been surrounded by ALL WOMEN! ALL THE TIME!) it certainly seems like something’s amuck. But then again, we have to give math credit where credit is due. I still kind of want to have shared spreadsheet where this can be tracked among my friends. Maybe that’s just me though.

So…I lived in dorms for eight years (high school AND college), and every year, I was living with at least twenty other girls–often more. I was also one of three women in my nuclear family, so I feel like I have plenty of anecdotal experience to pull from. Yes, occasionally, we were syncing up, but it wasn’t consistent enough to prove that cycles really do sync up. Since everyone’s cycles are different (some have 28 days, some have 31, etc.), my guess is that it’s more coincidence than anything else.

I also grew up w/three other females (and no males)–my mother and my two younger sisters. We were never on the same schedule. I started when I was 11 (Mom would have been 36) and my sisters started by 12 or 13; I would have been 16 or so at that point. None of us (that I know of) were on hormonal birth control. I left home at 18 1/2, and started taking the pill once I was in college. I lived in a suite w/three other women my freshman year–still no syncing. And so on.

I strongly recommend trying a different pill if the one you’re on is giving you a three week period. My sister has had multiple issues like that, and it took her awhile to find one that worked best for her. Hang in there! And make sure you’re getting plenty of iron so that you don’t start getting anemic.

I’ve been on it before and never had problems like this! The nurse at PP told me to go ahead and start taking it, even though my period was due in, like, three days. I’m thinking that when I start my new pack it will go away. Hopefully. Also, I didn’t even think about the iron thing. Good point.

Definitely talk to your doctor about it. I once had something similar going on — a three week period (I was on the pill but it wasn’t new to me)-  and I had an underlying issue that was causing the prolonged bleeding. A round of antibotics and I was back to as good as new.

I don’t know, but every year in college my period decided to match my roomie’s period (same one all 4 years).  It was nuts….and irritating.  I’d always go to her, which I suppose is fitting because she’s bossier one out of us.  But yeah, by November our periods would be within a day or two of each other.

That being said, I shared a house with six girls my senior year and our periods were not synced at all.

Maybe it’s completely coincidental but all my roommates (a total of 4 – so, you know, small sample size) had their cycles change within a couple months of moving in. Mine doesn’t change because I haven’t been off the pill since I started taking it at 18. One roommate said hers changed in two months. I think she was mad at me?

I don’t know. That’s just what they all said. I wasn’t one to track their cycles.

(None of them moved in while on the pill)

this is really interesting!
I lived in a dorm with 30 other women for three years, and there was always lore about how, by the end of February, we would have “synced up” into two distinct schedules, all based on which end of the hall you lived on. and depending on which end you lived on, you would “sync up” with the “most dominant woman” on that end.

Yeah, this piece and Dr. Kate Clancy’s on menstrual synchrony has me calling bullshit on the idea:

 the study of human menstrual synchrony has suffered from three major problems: first, whether a mechanism exists that can produce menstrual synchrony, second, methodological issues with existing papers and third, statistical artifacts in how one analyzes synchrony. A related issue is that, from an evolutionary standpoint, many struggle to find a reason women would want to have synchronous cycles, and an ancestral environment in which it could even happen.


Speaking of the evolutionary standpoint, check out this hilarious nonsense from Cosmo. The choicest bit:

It may be that back in caveman times, having a group of females be fertile all at once helped propagate the species, since one man could impregnate them very quickly.

NO. SO many ways this is wrong. It would be terrible for survival if every woman gave birth at the same time, and since we’re fertile once a month, it’s not like there would be any advantage to having everyone ovulate on the same day. That would be one tired caveman!

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