A crippling and life-threatening disease
Some people have regular periods, while others have cycles that are less regular.
If you have your period for one week out of the month, and you live with three other women, two of you will probably be having your period at the same time.
Some people experience bleeding for a week, while others only bleed for a day or two.
That's because cycle length varies from person to person. Some people have their period every 21 days, while others see their period begin every 35 days.
Menstrual synchrony might often appear due to the laws of probability more than anything else.
Due to the fact that each cycle is different, period syncing might to be a subjective metric that depends on how we define "syncing up."
One study found that women exposed to male pheromones saw an increase in the hormone responsible for triggering ovulation. But there still isn't much evidence that connects pheromones and period syncing.
McClintock believed that period syncing may have less to do with close female interactions and more to do with close female friends being exposed to male pheromones at similar times.
Period syncing is controversial because experts still don't know for sure if the pheromones McClintock theorized on can actually influence the start of your period.
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. It can be a very different experience for people who menstruate.
The results found that after three cycles, 273 of the pairs actually saw a larger difference in their cycle start date than when the study started. This means that period syncing didn't occur for these pairs.
It's important to note that the study by Clue and Oxford University wasn't peer-reviewed, which means that outside medical experts haven't signed off on the study's methodology or conclusions.
They analyzed the cycles of 360 pairs of women who knew each other well and tracked their cycles through the app.
In 2017, period tracking app Clue teamed up with Oxford University to better understand cycle syncing.
Other studies conducted in the 1990s found conflicting evidence about period syncing. A 1993 study found that 29 same-sex couples where both had periods didn't experience syncing.
The study found that, after six months, those who were in close proximity to each other had their periods start three to seven days apart, whereas the control group periods started five to 15 days apart.
Because the study lacked the additional cycle factors, other researchers found its analysis to be flawed, and many have since disregarded its findings.
Pairs with close proximity were grouped together as closest friends or roommates, and pairs that had no relationship were used as the control subjects.
A study from 1995 looked at pairs of close friends who didn't live together, and the results showed that they didn't experience period syncing either.
But the study didn't test other cycle factors, like when the women ovulated. However, it did track when the women's monthly bleeding began.
Martha McClintock, a researcher at Harvard University, analyzed the menstrual cycles of 135 women living in college dorms, aged 17 to 22.
The first scientific evidence of period syncing came about in 1971, with the McClintock study.
The idea of period syncing has been passed down from mothers to their daughters, and discussed with friends and family for centuries.
McClintock theorized that pheromones, natural chemicals that send messages from one person to another, could be linked to period syncing.
But do keep in mind that if your period is "out of sync" with the women you're the closest with, that doesn't mean anything is wrong with your cycle or your relationships.
Sources: (Healthline) (Insider)
Period syncing is based on the theory that when you come in contact with another person who menstruates, your pheromones influence each other so that, eventually, your monthly cycles line up.
Curious? Check out this gallery to discover what research has to say about period syncing.
Do women really influence each other's menstrual cycles?
Here's the truth about period syncing
HEALTH Women's health
If you've ever lived with multiple people who have periods, you've probably experienced period syncing. Period syncing describes a popular belief that women who live together or spend a lot of time together will see the timing of their monthly periods syncing up or aligning. Roommates, close friends, coworkers, same-sex couples, and family members who start their period on or about the same day every month can feel like this is a special bond between them. But is this a real phenomenon, or just a popular myth?
Curious? Check out this gallery to discover what research has to say about period syncing.