BBC Hereford & Worcester invited me on air to answer this question on 'menstrual synchrony' from a listener. Here is a summary (and slightly extended version) of my radio interview. (Scroll down to the end to find the resources I used to answer the questions).
Most menstruators will tell you that at some point or other they have mystically sync-ed with another menstruator they have spent time with.
This is known as 'the myth of menstrual synchrony' or, sometimes, the 'McClintock Effect' after Martha McClintock who, in 1971 (around the birth of the modern global menstrual movement - or, what I call, menstrualism) conducted the first scientific research into menstrual synchrony and found it to be true among 135 college menstruators who were sharing a dorm.
McClintock's research was hugely influential and taken, especially by the media, to be conclusive.
Further research was later conducted through a biological lens: looking into the release of pheromones as a way of prompting menstruation.
Other research suggested that it was evolutionary, that perhaps there was a time when women menstruated together in an image much like the one portrayed in the fictional work of Anita Diamant's Red Tent (which itself has sparked a movement of Red Tents - menstrual cycle peer support - in communities all over the world).
Some believed that menstrual synchrony was a mathematical coincidence - given the probability of cycles, regular and irregular, overlapping.
Some social scientists believed that menstruators wanted to be 'in sync' to help create a feeling of sisterhood and solidarity, removing the cultural shame and stigma away from menstruation.
Almost fifty years after McClintock's research, in 2017, the University of Oxford 'confirmed' the mathematical probability of menstrual synchrony. Using data from menstrual tracking apps, with 274 pairs of menstruators, it found menstrual synchrony to, in fact, not be present at all.
I do not believe the research on menstrual synchrony is yet conclusive: I do not think, for example, that the recent research had a big enough sample size and it didn't explore or compare the specific environmental, biological and sociological factors of the menstruators. In my own work I have noticed (anecdotally) a 'permission field' that seems to help bring on a period i.e. being in a safe space. To suggest it's mere mathematics undermines, yet again, I believe, the experiences of menstruators themselves. So, whilst it remains open to further investigation scientifically, I would rather continue calling it the 'magic' of menstrual synchrony rather than the myth of.
The key learning here, as menstruation scholar Breanne Fahs points out, is: How do we know we bleed at the same time as others?
Through talking about it.
So let's keep doing that, shame and stigma free. If we're in sync, let's support each other, if we're not, let's do the same and support those who are bleeding around us.
To answer this question, I re-read the brilliant:
Cycling Together: Menstrual Synchrony as a Projection of Gendered Solidarity, in Out for Blood: Essays on Menstruation and Resistance (Suny, 2016) by Breanne Fahs
I'm So Over The Moon... Myth (Part 1 & 2), www.menstrualmatters.com by Sally King