The 'sync up': it has long been a popular belief of anyone who menstruates. If you live with another person who also has periods, then chances are you've had a conversation to see if you've 'synced up' with one another.
The idea that periods synchronise if people spend a lot of time together - including family members, housemates, friends, and partners - has been around for a long time, and has been believed to be connected to pheromones.
The idea orginated from the 'Menstrual Synchrony and Suppression' study by Martha K. McClintock in 1971, which introduced the claim that in a group of people who menstruate, one 'alpha uterus' will influences others around it to ovulate and menstruate in unison due to it's 'strong hormonal pull'.
The study was published in a journal called Nature, and recorded data on the menstruation patterns of college students living together in a dormitory in the US. The study reported an increase of synchronisation for the pairs of women who would spend more time together, or were paired up as room mates.
There's been serious doubt cast over this study since. A number of studies have failed to replicate the initial findings (both within humans and animals), and mathematical analyses show that some degree of syncing is to be expected - so, an overlap of cycles from the women in the study likely came down to chance.
There's been no conclusive scientific data proving the existence menstruation synching ever published.
Recently, popular menstruation-tracking app Clue ran a self-funded user study about the phenomenon - their findings were sent out to users, stating that results showed no signs of the 'sync up' being real.
On behalf of the app, researchers reviewed data from 360 pairs of people (of various different types of relationships), and analysed a minimum of three consecutive cycles for each pair.
Of the 360 pairs, 273 actually had larger differences in cycle start dates at the end of the study compared to the beginning.
Seventy-nine pairs experienced their cycle start dates getting closer together over consecutive cycles, and only 19 of the pairs within this group lived together.
So, according to the statement made by a spokesperson for Clue, it seems as though the opposite of synchronisation occurred for the majority of those studied: "From our small pilot study of Clue users, our data scientists found that cycles between pairs and cohabiting individuals did not align. Our statistical evidence also indicated that cycles are actually more likely to diverge, rather than sync, over time."
"Our statistical evidence also indicated that cycles are actually more likely to diverge, rather than sync, over time."
Yes, the not-so-fun things about menstruation would be far easier if your cycle occurred simultaneously with your friends - you'd have endless solidarity and sympathy, plus you'd have someone to remind you when your period is about to begin (so, no nasty surprises or ruined underwear). But unfortunately, it seems as though conclusive independent evidence still eludes us.