Period syncing myth debunked

Menstrual cycles do not actually sync up between women who spend a lot of time together, new research has found.
Despite the fact that no scientific studies have been able to prove the theory that those with uteruses can experience synced cycles, the idea has become almost fact. However, a new joint study performed by period tracking app Clue and Oxford University has debunked the myth of syncing periods.
Clue sent users of its app a survey asking about their experiences of synced periods with other app users. The poll included questions about their relationship, whether they live together and what hormonal birth control is used by both, if any.
Out of 1,500 responses, there were 360 pairs of users who had at least three cycles over the same time period that could be reviewed. Researchers analysed a minimum of three cycles per pair, revealing that 273 of the pairs actually had larger gaps between the start and end dates of their cycles at the end of the study compared to its beginning.
Of the 360 pairs analysed, just 79 had cycles that featured start dates getting closer together throughout the course of the study.
For the full sample used, the average difference in terms of the start of a cycle at the beginning of the study was ten days. This increased to 38 days at the end of the study, showing that periods did not sync.
The study also showed that living together did not increase the likelihood of periods syncing. Some 37 per cent (100 out of 273) of the pairs with cycle start dates that were different lived together, whereas just 24 per cent (19 out of 79) of those with converging start dates lived together.
Not only did the study find that periods will not sync over time between those who spend a lot of time together and those who live together, it actually revealed that they are more likely to become more separate.

Is it true that periods synchronise when women live together?

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It’s commonly believed that women who live together eventually experience the syncing of their monthly periods. But could it just be chance?

The theory behind the syncing of menstrual cycles is that women’s pheromones interact when they are in close proximity, causing them to have their period at the same time. Many females buy into it.

“I definitely think it’s true,” says Emma. “It would be too much of a coincidence otherwise.”

Emma, 24, lived with five girls at university. She says that within a few months they all had their period at the same time.

They were all buying tampons at the same time, they were moody at the same time – and their one male housemate really noticed it.

“It’s a popular belief,” says Alexandra Alvergne, associate professor in biocultural anthropology at the University of Oxford.

“As humans we always like exciting stories. We want to explain what we observe by something that is meaningful. And the idea that what we observe is due to chance or randomness is just not as interesting.”

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Alvergne has tried to review the evidence for this phenomenon. She says the idea started with research findings published in Nature, a scientific journal, in 1971. A researcher called Martha McClintock studied the menstrual cycles of 135 women in an American college.

” found that the onset of the date of menstruation was more similar among friends and roommates than among random pairings of women,” says Alvergne.

Dr McClintock hypothesised that this was because the women who were spending time together had the chance for their pheromones to affect each other.

And why would this happen? The most prevalent theory was that it was an evolved strategy among females to co-operate with each other – to stop becoming a kind of harem for a single dominant man.

The idea is that if women had synchronised cycles, they would all be fertile at the same time – so one man would not be able to reproduce with them all.

“He can’t manipulate all the females at the same time so that’s why it was believed to be a form of co-operation between females,” says Alvergne.

Image copyright Getty Images Image caption Did the idea of synchronised menstruation reflect the emergence of the feminist movement in the 1970s?

In the 1970s when this paper came out, feminism was emerging as an important movement. Alvergne thinks this may be one of the reasons the idea became so popular.

“I find that sometimes ‘society values’ are hiding in hypothesis. And from a feminist point of view, the idea that females would co-operate in the face of male domination is attractive.”

Other studies, some in humans and some in other primates, seemed to show similar results.

But, there were also studies that didn’t find evidence of periods syncing. And people started to pick holes in the earlier research – critics identified problems in the people chosen for the study. The definition of when they were syncing was quite loose.

Perhaps most importantly, critics pointed to the fact that in McClintock’s 1971 findings she didn’t account for chance to explain why women’s cycles were in sync.

This seems to go against what many women feel they experience.

Image copyright Science Photo Library Image caption That syncing feeling – but it’s probably just a feeling

“If I spend any period of time with a woman who is menstruating, I will start my period,” says Inez, 26. “My womb is a huge team player. It just wants to be one of the girls.”

Not only that but Inez thinks some women have wombs that prefer to play a leadership role.

“I’ve got a classic beta womb. My friend Suzanne has an alpha womb. She will bring anyone on within a 10-mile radius. She walks in menstruating, everyone is reaching for the tampons.”

But the problem comes when the theory seems to break down. Inez says she has never synced with her flatmate.

What are periods?

  • A period is the part of the menstrual cycle when a woman bleeds from her vagina for a few days
  • For most women this happens every 28 days or so, but it’s common for periods to start sooner or later than this, ranging from day 24 to day 35
  • Periods can last between three and eight days, but will usually last for about five days

Source: NHS Choices

“I have known her since I was 12 and I think she is quite offended that I don’t sync with her.”

Alvergne is not surprised that women observe overlap in their menstrual cycle with their friends.

“The question is – does it happen by chance or not? By chance you would expect it to happen at least half of the time.”

Some academics recently decided to find out if the findings of synchronicity of periods could be put down to chance. They looked at six years’ worth of data of the menstrual cycles of our close cousins, baboons.

“They proposed two models,” says Alvergne. “One was the model with the exciting hypothesis” also known as the “evolved strategy” in which women sync as a defence against dominant males.

“And the other model was the boring model. Where the patterns are explained by chance.”

The researchers compared how much each model would account for the data being observed. They found that the model assuming that patterns would appear by chance was the best model by far.

More research could be carried out in the future that does reveal evidence that women’s periods sync. But currently many researchers are sceptical.

“Maybe actually, what we observe is nothing more than randomness,” says Alvergne.

Image caption The menstrual cycles of the four women fall into sync in June, but fall out of sync again from July onwards

Additional reporting by Elizabeth Cassin

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What about corroborating stories? Menstrual cycle length can vary from woman to woman, as well as be affected by a variety of factors, such as sleep, stress and fluctuations in weight. So the chances are two women who spend time together will have an occasional menstrual cycle that starts within one or two days of the other. Remembering these seemingly synced periods as a consistent trend rather than chance is recall bias. Women are probably less likely to take note of all the time they routinely spend in the close company of other women — spouses, partners, roommates, co-workers or friends — whose cycles do not coincide.

There are a variety of period tracking apps — Apple Watch just debuted a new feature for menstrual cycle tracking. A 2016 review of apps on the market at the time found many were inaccurate or contained misleading health information. While these apps may allow for greater ease of entering and recalling data about cycles than a calendar (digital or paper), unless they have been cleared by the Food and Drug Administration, relying on these apps as a health tool is a buyer (or user) beware situation.

Whether you just got your first period or you’ve had it for years, it can be totally nerve-wracking when your period is late or irregular. Your mind might immediately jump to pregnancy (if you’re sexually active) or a serious medical condition (if you’re not). But there are actually a bunch of different reasons why your period might be out of whack. We talked to Jennifer Ashton, M.D., a New Jersey-based gynecologist and the author of The Body Scoop for Girls: A Straight-Talk Guide to a Healthy, Beautiful You, about the different factors that can cause your period to get off track and when you should be concerned.

Here are 10 possible reasons you’re not getting your period on the regular.

1. You started menstruating within the past two years.

Puberty is a gradual process. You don’t go to sleep in a kid’s body and wake up in an adult’s body the next morning, right? Similarly, the signals that travel from your brain to your ovaries that induce your period each month take awhile to mature. Ashton calls your first cycles “training wheels” for the next couple of decades. “If the period is irregular in the first two years, we usually don’t get worked up over it because it’s so common,” she says.

Most girls get their periods between the ages of 8 and 16, although getting it earlier or later than that isn’t necessarily cause for concern. Not every irregular period is an urgent problem. The younger you are, and the more recently you’ve begun menstruating, the less likely there’s any cause for concern — your body just takes a while to get used to its new habits. But regardless of what your period is like, Ashton recommends visiting a gynecologist for your first appointment between the ages of 13 and 15. You won’t necessarily need a physical exam, and it’s a great opportunity for you to ask your doctor any questions you have about your reproductive health.

2. You’re tracking it wrong (or you’re not tracking it at all).

If your menstrual cycle is healthy, the first day of one period and the first day of the next period should be between 21 and 40 days.

If you don’t track your period, there’s a chance you might not even notice you’re irregular. It’s tough remembering exactly when you got your last period (was it the day of your chem quiz? or did it start the day of your history quiz?), so if you don’t use a period tracker app on your phone already, download one now to best stay on top of your health. Period Tracker Lite is free on iOS and Android, and it’s easy and discreet to use.

3. You’re underweight.

While there’s no minimum weight or body mass index (BMI) required to menstruate, you do need a certain percentage of body fat in order to have regular periods. “You can think of that very logically from an evolutionary standpoint,” Ashton explains. “If a girl is underweight, her brain thinks she’s in a situation of famine, so it’s not the best time to reproduce.”

You might have heard that highly athletic girls with low body fat sometimes lose their periods. While this is biologically possible, most girls who are on their school’s sports teams or working out at the gym won’t experience this unless they’re doing something extreme, like training for a marathon or triathalon.

If you suffer from an eating disorder or are concerned that your weight may be interfering with your cycle, talk to your doctor.

4. You have polycystic ovarian syndrome (PCOS).

PCOS is a disorder that causes your body to make the right types of hormones, but in the wrong ratios. Doctors aren’t sure if it’s caused by genetics or if there’s a specific trigger, but girls with PCOS often have periods that are very spaced out — six to eight weeks apart, or even longer — or irregular for more than a year after their first cycle, because their hormone imbalance causes overactivity in the ovaries. Other symptoms of PCOS include weight gain, oily skin or acne, depression or mood swings, and hair loss or hair growth. If you happen to have any of these symptoms, you don’t necessarily have PCOS, but if you experience a few or more and are concerned you might have it, talk to your doctor.

5. You just started taking a form of hormonal birth control.

The Pill, the patch, the NuvaRing, the Depo-Provera injection, and most types of the IUD are all forms of hormonal birth control, which changes the levels of estrogen and progestin in your body that affect your period. It can take awhile for your body to adjust, especially if your cycle was irregular to begin with.

In addition to lowering your risk of pregnancy, hormonal birth control is often prescribed to girls with really irregular periods to help make their cycles more predictable and comfortable. But because every body is different, it can take some trial and error to figure out which form of hormonal birth control works best from you.

“Most birth control pills (if not all) will ‘regulate’ a period in time. It’s just a matter of finding the right pill that works for that particular girl,” Ashton says. Non-hormonal forms of birth control like condoms, by the way, do not involve hormones and therefore, won’t affect your period.

By the way, hormonal birth control does not completely eliminate your risk of pregnancy. Even with proper use, it still has an eight percent failure rate per year, according to Ashton. To reduce your risk of pregnancy further, you can use a condom in addition to taking the Pill.

6. You’re not taking the Pill properly.

Birth control pills come in two types: monophasic, where every pill in the pack is the same, and triphasic, which has a variety of active pills (containing different types and levels of hormones) and placebo pills (which don’t include hormones). If you take triphasic birth control, you bleed during the week you take the placebo pills. But if you skip the placebo pills, you won’t bleed.

It’s also important to take the Pill at the same time every day, without missing a day. “If girls miss pills or skip days or take it at widely variable times during the day, that can cause irregular bleeding,” Ashton says. It can also make the Pill less effective.

Need clarification on how to take the Pill properly? Talk to your doctor. Remember, there are several different types and brands of the Pill, so you need to be sure you’re taking it as directed.

7. Your cycle is syncing up with another girl’s.

If you live with other girls and women who menstruate, like your mom, a sister, or a roommate, there’s a good chance you’ll eventually wind up on the same cycle, Ashton says. Scientists don’t know why this happen, but it’s incredibly common — and it explains why you might get your period on a day you didn’t expect.

8. You’re taking certain medication.

If you’re taking meds for depression, anxiety, bipolar disorder, or another psychiatric disorder, they could potentially disrupt your cycle. “The more medications you’re on, the more potential there is for irregular bleeding,” Ashton says.

This doesn’t mean you should go off your meds without consulting your doctor. Ask your doctor about your options — you might be a good candidate for hormonal birth control, like the Pill or an IUD.

9. You’re experiencing vaginal bleeding, but not because of your period.

Not every instance of vaginal bleeding is a result of your period. In extremely rare cases, you might bleed due to an infection, a tumor, a bleeding or clotting disorder (such as Von Willebrand Deficiency), or a foreign object inserted into your vagina. One way to determine if you’re bleeding because of your period or for another reason is to track your cycle, but if you have concerns, speak with your doctor.

10. You’re pregnant.

Missing your period is one of the earliest signs of pregnancy. If you’ve been sexually active and miss your period, you might be pregnant. No matter how vigilant you’ve been about birth control or condoms, there’s always a small risk of pregnancy. If you think you might be pregnant, purchase a pregnancy test at a drugstore or a health clinic (like Planned Parenthood).

Ashton also cleared up some common misconceptions about irregular cycles: For the average teen girl, stress, poor diet, and frequent travel or exercise will not affect your period. Remember, if you’re concerned about what is or isn’t going on with your cycle, talk to your doctor.

And if you’ve ever wondered what NOT wearing a tampon or pad during your period would be like, read this brave gal’s tale of free bleeding. Happy periods, all!

Cut back on caffeine and alcohol

Both alcohol and caffeine can increase cortisol levels, so it’s usually recommended that you reduce your intake of both of these when dealing with major life changes, going through a rough patch, or nearing your menstrual cycle. Instead, you might try decaffeinated beverages or herbal teas that are known to have calming effects, such as chamomile or lavender.

Prioritize healthy sleep

Getting better sleep is often one of the best ways to overcome stress. Often we focus on the quantity of sleep, but the quality is also of great importance. Most people need about 7-9 hours of sleep to really refresh themselves. Sticking to a sleep schedule and routine can help improve your quality of sleep.

Avoiding screens for a few hours before you go to bed, using a sleep mask, and a white noise machine can all make falling asleep and staying asleep easier. This will allow the body to fall into a proper sleep rhythm which can lower the chance of insomnia.

How to prevent stress from delaying your period

The first step to prevent stress from delaying your period is to understand what’s causing your stress and how much stress you can manage. You may not always be able to avoid stress, but you can develop healthy ways to cope with it. Tracking your cycle and any changes you experience in your moods will make it easier to identify any issues that may arise so you can better understand why your period is late.

While stress is a common cause for a late period, it is just one of many potential reasons for a delay in menstruation. Pregnancy, hormonal birth control, and health problems like polycystic ovary syndrome (PCOS) can also make your period late.

Sometimes the stress of a potential unwanted pregnancy can make your period late. Taking a pregnancy test to find out if you are pregnant can reduce this stress. If your period is late, and you’re experiencing symptoms like unwanted hair growth, headaches, weight gain, and difficulty sleeping, you may want to see a doctor to determine if you have PCOS, which is a treatable condition.

Tracking your mood, life events, and symptoms in an app like Flo can help you gain perspective on your level of stress, and taking simple measures like exercising or making time for meditation can help you get your period back on track.

Do 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!

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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.

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Myth busting: do women’s cycles really sync up?

Ever lounged on the couch, eaten ice cream, and fought over the heating pad with your roommate? Or perhaps you never need to worry about having ibuprofen or your favorite essential oil on hand, because you know your coworker will also have some around a certain time of the month.

According to a 1999 study, 80 percent of women believe in period syncing and 70 percent reported that they enjoy when it happens to them. But as much as we want to believe that our periods do sync up (there’s something about the camaraderie it creates), we can’t help but wonder if it’s actually true. Is there scientific evidence or data that supports cycle synchronization when women spend a lot of time together?

For over 40 years, researchers and doctors have been trying to figure out the answer, and they’ve had little luck—until recently. Here’s what the scientific community has learned over the years and what they discovered in 2017.

The study that started it all

In 1971, Harvard doctor Martha McClintock studied a group of 135 women living in a college dorm. Their research found that over the course of the school year, there was an increase in period synchronization for roommates and close friends, but not among random pairings of women.

McClintock hypothesized that this was due to time spent together, eating meals together, experiencing stress together, or even a theory called the “alpha uterus” (seriously). According to the study, an alpha uterus has a “strong hormonal pull that causes other cycles around it to menstruate in unison.” She proposed that when a woman becomes aware of another woman’s period (by hearing her mention it or seeing her carry a pad to the bathroom), it might cause them to start her period. The study suggested that these things can cause women’s pheromones to communicate with each other due to physical closeness. This, in turn, triggers cycle syncing.

A series of mixed findings

Since 1971, multiple studies have been conducted on female roommates, best friends, lesbian couples, and even animals to see if menstrual cycles actually converge. But nearly all of these studies have mixed findings and two in particular fail to replicate the 1971 study’s results. A 2006 study, for example, collected period data from 186 Chinese women living in a college dorm together, and the results showed that the women’s cycles didn’t sync up at all.

In a 1998 study, McClintock, along with Kathleen Stern, announced that they had “definitive evidence” that human pheromones could manipulate the timing of ovulation. Beverly Strassman, professor of anthropology at the University of Michigan, used their assertion to inform her 1999 paper in Human Reproduction, arguing the opposite:

“If the menstrual cycle is 28 days long, then 14 days apart would be maximal asynchrony. By chance alone, one would expect two women to be 7 days apart (half of 14 days). Given that a menstruation can last 5 days, overlapping periods are a common occurrence. That women synchronize to each other, however, is a myth.”

The latest? Cycles don’t converge — they actually diverge

This past year, the period and fertility tracking app Clue, in collaboration with a doctor from Oxford University, tried to find the answer by taking a more data-driven approach.
The data science team surveyed Clue users and asked if they thought their cycle had been syncing with another woman’s, who also uses the app. Then, they asked the users what type of relationship they had with this other person (friends, siblings, partners, roommates, coworkers, etc.), if they live together, and if they’re on hormonal birth control.

The Clue team received over 1,500 responses and narrowed it down to 360 pairs of users whose cycles occurred during a similar time period. From there, Clue tracked three consecutive cycles for each pair. Here’s what they found:

“273 pairs (76 percent of the sample) actually had a larger difference in cycle start dates at the end of the study than at the beginning of the study. Only 79 of the pairs behaved in the opposite way, with the gap between cycle start dates getting closer during the study. For all 360 pairs, the average difference at the beginning was 10 days, and 38 days at the end.”

Based on this study, your cycle is actually more likely to get out of sync than in sync with another woman’s period.

Our take

The Clue study is the most data-driven and has the largest sample size compared to the others, indicates that it’s unlikely that women’s cycles sync. But it’s important to keep in mind that this was still a relatively small study. There’s more research to be done on this topic and in women’s health in general—something that’s unfortunately been overlooked for decades—until we know for certain.

Even though the research says women’s cycles don’t converge, that doesn’t mean you can’t enjoy the feeling of comfort, connection, or sisterhood that comes along with having your period at the same time as someone else. So, keep knowingly nodding when a sister asks to borrow a tampon. While your cycles may not be biologically syncing, you’re still going through the same thing.

Do Women’s Periods Really Sync Up When They Spend Time Together?

Period syncing is the long-held belief that when women spend a lot of time together—in the dorm, at the gym, at work, at home—their menstrual cycles will inevitably sync up. Do any of these scenarios sound familiar?

“I am one of three girls. My mom, my two sisters, and I lived together until I left after college. Our periods were completely synced up during that time,” Ashley says.

“When I was a drill sergeant, we had a female platoon. By the end of basic training, many of the troops were on the same cycle,” Terry says.

In one study, 84 percent of women were familiar with menstrual synchrony, and 70 percent said they’d experienced it in their own lives. But, according to science, period syncing doesn’t happen as a result of being close. Here’s why.

How the Buzz Began

Period syncing officially entered the science world in a 1971 article published in the journal Nature. For the study, researcher Martha McClintock tracked the menstrual cycles of 135 college-age women living in the same dorm. McClintock’s results suggested that the onset dates of the women’s periods were more likely to become closer together if they were roommates and/or good friends.

She wrote, “Although this is a preliminary study, the evidence for synchrony and suppression of the menstrual cycle is quite strong, indicating that in humans there is some interpersonal physiological process which affects the menstrual cycle.” McClintock theorized that perhaps pheromones—chemicals the body secretes—or simply awareness of close friends’ and roommates’ cycles could cause changes in the start date of a woman’s cycle.

Women seemed to really dig this idea because many had noticed period syncing in their own lives. And they were experiencing it in solidarity with each other.

Debunking the Myth

But here’s where science debunks the idea of period syncing. Later research revealed that McClintock’s research had errors. When the errors were corrected, no statistically significant evidence of period syncing was found. Other studies have been performed, but none have proven McClintock’s theory.

Most recently, in 2017, the period-tracking app, Clue, partnered with Alexandra Alvergne, Ph.D., from the University of Oxford to study period syncing yet again. They tracked the menstrual cycles of pairs of women who were close to each other. They actually discovered that their periods were likely to get farther apart, not closer together, in timing.

“ is a myth that certainly needs to be debunked,” says Angela Jones, M.D., F.A.C.O.G., an OB-GYN in New Jersey and sexual health adviser for Astroglide. “There is no evidence that supports it.”

Why Some Women Swear It Happens

If there’s no such thing as period syncing, why do some women swear their cycles become aligned with their friends’, sisters’, or coworkers’?

“Due to the limited amount of days in a month, the fact that women have different cycle lengths, and that women’s periods last different amounts of days, there is bound to occasionally be some overlap,” Jones explains. “But this has nothing to do with pheromones or other factors causing women that live together to have synchronized periods. It is simply chance.”

Think of it this way: Some women have regular periods, and others don’t. Some have longer cycle lengths, and others have shorter ones. Plus, there are several proven factors that can affect the onset date of a woman’s cycle. These include going on or off birth control, stress, and chronic medical conditions, Jones notes. There are only 28 days in the average cycle. A few days of change here and there, and eventually you’re going to have your period at the same time as your BFF. (Though it probably won’t stay that way forever.)

Of course, we’re going to notice that our friend says she’s craving chocolate or asks for a tampon at the same time we’re feeling cramps. We’re not going to notice when it’s not happening.

So, yes: You and your close friend or gym buddy may get your period at the same time. But you can’t chalk it up to science. Only coincidence.

You’ve probably heard that women who live or work together tend to wind up on the same cycle. Friends have told me how their coworkers all seem to complain of menstrual cramps at the same time, and the tampon box in the women’s restroom empties regularly each month like clockwork. And books such as The Red Tent popularize the belief that our menstrual cycles sync with the women close to us.

While it’s often stated as a fact, you might be wondering: Do women’s periods actually sync up? Or is it merely the stuff of urban legend?

Past Findings

Studies on women’s menstrual cycles and how they did or didn’t sync have been conducted for decades. One of the earliest examples is a 1971 study widely credited with “proving” that women who live together or are close friends tend to be on a similar cycle and attributed this to chemical signals given off by the body known as pheromones. However, the study’s statistical design was criticized because while a few researchers were able to replicate the results in future studies, other researchers were unable to replicate them. Even though the study itself had holes, the myth has persisted.

The Research Today

More recently, researchers used data from the period tracking app Clue to investigate whether there is evidence for this phenomena. The study looked at 360 pairs of women who used the app and who reported that their cycle was syncing with someone they knew who also used the app. After analyzing the data, the researchers found that 273 of the 360 pairs actually experienced their cycles differing rather than syncing. Only seventy-nine of the pairs saw their cycle start dates get closer to one another. The researchers found that the women’s cycles were more likely to diverge over time rather than converge. They also found that living together did not increase the likelihood of women’s periods syncing.

Blame the Coincidence on Our Cycle Lengths

In an interview with Scientific American, researcher Jeffrey Schank observed that because women’s cycles are often different lengths, it’s difficult for them to occur at the exact same time. He theorized that any similarity in cycles between women is likely more random and less likely to remain the same over time because of the small shifts in their cycle lengths. And, in his own study—the longest done on menstrual synchrony to date—he found no evidence of periods syncing up. Instead, Schank and his team found only random overlaps. Given that there are so many factors involved in the length of a woman’s cycle and her fertility (e.g., miscarriages, pregnancy, postnatal, stress, exercise activity, weight loss or gain, and nutrition), it’s easy to see that there is a great deal of variability in an individual woman’s cycle that would make it difficult for her menstrual cycle to synchronize consistently with the women she is in close contact with.

So as it turns out, more and more research findings point to evidence that women’s periods don’t actually sync, and anytime your cycle coincides with a friend’s, it’s most likely due to chance. Our bodies are as complex as our menstrual cycles. While it would be a cool (and perhaps useful) phenomenon, it would require the stars perfectly aligning for your period to consistently sync with your female coworkers, relatives, and closest girlfriends. But don’t worry, there are plenty of other things you can bond over with the women in your life!

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