Can periods really sync?

Is there any truth in the possibility that menstrual periods can ‘sync up’?


Emma Thornton
Qualified Nutritionist (ANutr)
@EmmaThornton
Ask Emma


15 November 2016

What is this idea of synchronising periods?

Whether or not menstrual periods are able to ‘sync’ is still up for debate by many. This idea suggests that women living in close proximity to each other, for example roommates, siblings, mothers and daughter or close friends, are able to fall into menstrual synchrony. That is, that their menstrual cycles align so that they find they are having their periods at the same time each month.

I’ll be discussing this topic, how the idea first came about and what evidence there is to support it.

What theories exist for periods syncing up?

This idea of the menstrual cycles of women living together syncing, has been around for some time. Some claim that it’s all part of evolution – so, let’s consider what purpose this might have served.

If a group of females are living together and have synchronised periods, then they will also be at their most fertile around the same time, right? Ok great, so this means if a strapping young male comes along, he could potentially impregnate a large number of the females around the same time. However, does this really make sense? What if the male shows up at the wrong time – when all the females are on their period – then there’s a problem. Wouldn’t it be more beneficial to have some diversity and have a number of females all at different stages in their menstrual cycle so that they aren’t putting all of their eggs in the one basket? Excuse the pun.

Yes, so whether or not this theory makes much sense if up for debate, although, it hasn’t been completely ruled out just yet. What if the male isn’t so likely to show up at the wrong time, as he’s drawn to ovulating females by the presence of something called pheromones?

This leads nicely into another theory attempting to explain how periods could potentially sync up. Pheromones are chemicals that animals (and humans) produce that can influence the behaviour of others nearby. They are known to play an important part in initiating mating behaviours in some animals – many of the impressive mating rituals across the animal kingdom incorporate the use of pheromones.

But, could pheromones also have an impact on the menstrual cycle? Rather than assisting communication between males and females, is it possible that the release of pheromones between groups of females could somehow influence menstrual cycles and cause them to fall nicely into sync? It’s certainly possible and this idea has been subject to a trial or two over the years.

A couple of other theories have been thrown into the mix too, such as the proposal that periods fall into sync with phases of the moon. Surely this would mean that women over a wide stretch of area would all have their period at the same time though... Yes, that one’s perhaps a little more sketchy.

So what’s the evidence?

So the theories are there, but what evidence do we have to go on?

A pivotal study in this field was first carried out in 1971 by a scientist called Martha McClintock [1]. She aimed to study over 100 females living in close proximity in dormitories together during their time at college. The idea for this research came about as a result of her own experience during her studies and she hypothesized that the menstrual cycles of these women would synchronise and that pheromones and social interactions could be behind it all. Results from her study showed that over a period of roughly 8 months, close friends who had periods initially 8 or 9 days apart, ended up with periods only 5 days apart on average by the end of the experiment (in comparison to a control group of girls who lived separately and had their periods remain 10 days apart). She concluded that menstrual synchronisation was apparent.

Since this original study almost 40 years ago, flaws in its method have been recognised and subsequent studies haven’t been quite so convincing. A similar study carried out in 2006 [2] concluded that the menstrual cycles of Chinese students living together didn’t synchronise. The authors suggested that irregular menstrual cycles and the effects of chance are likely to be having an effect.

This proposal that period syncing is actually down to chance has cropped up in much of the more recent research and it may well be the cause of many of the supposed instances of menstrual syncing, it is thought some of the earlier studies didn’t properly adjust for this.

What’s the verdict then?

So, it seems that science is leaning towards it all just being a big myth. I admit that the variability in periods has a lot to answer for and outcomes can be misleading as a result of this. Throughout A.Vogel Talks Periods, I talk extensively about how periods are so inconsistent across the board and how no two women generally have the same experience! Menstrual cycles can range from 21 to 36 days and yet in many cases, still be relatively regular each month. Then, there are the many women who have irregular periods.

If we consider the scenario in which a mix of women are all living together in close proximity, unless they all have perfectly regular periods with matching cycle lengths, then it’s inevitable that their cycles will eventually catch up with each other and overlap. This could easily be mistaken for syncing. In real life it’s perhaps a bit too easy to notice syncing periods and forget about the mismatched months in between! Then you have the effect of chance which even some studies have failed to account for.

In terms of references, there isn’t a vast amount of research on this topic, and there does seem to be quite a bit of anecdotal evidence out there in its favour. This idea that periods could sync up in certain circumstances doesn’t seem to be dropping off the radar anytime soon and whether or not this is justified is still somewhat up for debate.

It’s important to also consider modern lifestyle factors; diet and exercise regimes, stress and sleep habits as well as patterns of taking hormonal contraceptives are important. These too could be having an influence on menstrual cycles and women living together often follow similar routines.

It is quite a nice, empowering thought I guess, that women can somehow control their cycles and stand stronger together. Although I have to admit, when considering the research, it’s pretty likely that that’s all it is. However, a few more up to date studies in this field wouldn’t hurt as there are some inconsistencies there and we mustn’t dismiss the anecdotal evidence out there – it can be a powerful tool.

[1] McClintock, M. (1971) Synchrony and suppression among a group of women living together in a college dormitory suggest that social interaction can have a strong effect on the menstrual cycle. Nature, 229, 244-245

[2] Yang, Z. And Schank, J.C. (2006) Women Do Not Synchronize Their Menstrual Cycles. Human Nature, 17(4), 433-447

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