Women who live together or spend a lot of time together begin menstruating on the same day every month is what known as period syncing and this is what mist women have experienced.

The question comes "Is Period Syncing a real thing?"

A 2017 study keeps the idea of period syncing alive by pointing out that 44 percent of participants that were living with other women experienced period synchrony. Period symptoms like menstrual migraine were also more common in women living together. This would indicate that women might influence each other’s periods in ways beyond the timing of their menstruation.

*But Synchronity is still hard to prove*

The truth is, we might never nail down how real the phenomenon of period syncing is, for a few reasons.

Period syncing is controversial because we don’t know for sure if the pheromones on which the theory hinges can influence when your period starts.

Pheromones are chemical signals that we send to the other humans around us. They signify attraction, fertility, and sexual arousal, among other things. But can the pheromones from one woman signal to another that menstruation should take place? We don’t know.

Period syncing is also difficult to prove because of the logistics of women’s period cycles. While the standard menstrual cycle lasts for 28 days — lots of people don’t experience periods that way.

Cycle lengths up to 40 days are still within the realm of what’s “normal.” Some women have shorter cycles with only two or three days of bleeding. That makes what we think of as “period syncing” a subjective metric that depends on how we define “syncing up.

Sources - healthline.com