Can periods really sync with your friends? Here's what science says

Ever noticed your period arriving at the same time as your best friend's? Doctors say what looks like menstrual syncing may actually be coincidence driven by natural cycle variations

Ever noticed that your period seems to arrive around the same time as your roommate’s or best friend’s? It’s a story many women swear by. College dorms, shared apartments, office friendships, sooner or later someone says, “Our periods have synced!”

 

But is this a fascinating quirk of female biology, or simply a coincidence? A gynecologist explains what science actually says about menstrual syncing, and why people believe this myth. 

What do people mean by 'period syncing'? 

The idea of menstrual synchrony is that women who spend a lot of time together, such as friends, roommates, or colleagues, begin to have their periods around the same time.

 

 

Dr Anjali Kumar, Director of Obstetrics & Gynecology at CK Birla Hospital, Gurugram, explains that there is a lot of natural variation between the menstrual cycles of different females and over different months which makes the task of closely pinpointing cycle completions extremely difficult.

 

According to her, the belief in period syncing became widely known after studies in the 1970s suggested that women living together began menstruating at similar times. However, later research began to question those findings.

 

“Menstrual synchronisation is unlikely to be a real biological phenomenon,” says Dr Kumar. “The first studies to prove this phenomenon were in the 70's and were proven to collapse due to poorly chosen samples and were unfounded.”

 

The patterns seen in those early studies are better explained by natural variation in menstrual cycles rather than a biological mechanism that links women’s cycles together, she further explained. 

The role of pure coincidence 

Menstrual cycles can vary widely, typically anywhere between 21 and 35 days, and they can shift from month to month.

 

That alone makes overlaps inevitable.

 

Dr Kumar says, “Any given month or cycle can appear to be aligned, especially given the average cycle days, and then will likely differ again.”

 

Researchers often analyse cycles over long periods using statistical models. What they usually find is that apparent syncing often disappears when observed over time.

 

“Most instances warranting the explanation of cycles being in synch are cases of statistical coincidence,” Dr Kumar says.

 

“In the past, pheromones or other chemical signals were thought to be responsible for the synchrony in cycles for women living in close quarter situations. The evidence supporting these claims have proven to be inconsistent and quite weak.”

 

She adds that while reproductive synchrony exists in the animal kingdom, scientists have not identified any reliable pheromone-driven mechanism that causes menstrual cycles to align in humans.

 

“Sometimes, the shared living situations of women can alter menstrual cycles as a result of stress, sleep, and eating, which can cause hormonal regulation to alter,” she explains.

 

If several people experience similar lifestyle changes at the same time, say, exam stress in college or late nights at work, their cycles may shift in similar ways.

 

That can make it look like synchrony, even though each body is responding independently.