We've all heard it: when women spend a lot of time together, their menstrual cycles begin to synchronize. We have seen it with friends, roommates or colleagues, and many girls believe this theory.
But is it just a coincidence?
Emma, ??24, tells BBC News that when she lived with five other girls during university, within a few months they all had their periods at the same time. "It would be very strange if it happened by chance," she says. "We bought tampons at the same time and everyone could sense the change in our moods, even our single male roommate."
Where did the idea of ??synchronizing cycles come from?
One of the first studies to mention this phenomenon was published in 1971 in the scientific journal Nature. Researcher Martha McClintock analyzed the menstrual cycles of 135 women at an American college and found that roommates were more likely to have their periods at the same time than women who didn't spend much time together.
McClintock hypothesized that the cause was pheromones—those chemicals the body releases that influence the behavior of those around it.
But why would evolution create such a phenomenon? An interesting theory suggests that synchronizing was a way for women to protect each other from the dominance of a single man. If they all became fertile at the same time, one man could not have control over them all.
What do scientists say today?
Although many women believe that menstrual cycles synchronize, some researchers are skeptical. Alexandra Alvergne, lecturer at "Oxford University", says that people tend to look for interesting stories to explain what they see.
"It's more exciting to think that synchronization is a biological phenomenon than to accept that it could just be coincidence," she says.
Why do we feel like our periods coincide with those of our girlfriends?
Alvergne explains that women's menstrual cycles can have some overlap, but that doesn't mean they're completely synchronized. Because a cycle typically lasts between 21 and 35 days, coincidences are inevitable.
After all, science still does not have a definite answer to this question. But one thing is certain: for some women, the idea that they share the same biological rhythm with their girlfriends gives them a sense of connection and solidarity - even if it's just a coincidence.
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