Is Period Syncing Real?

Most people with a period have a story about spending so much time around a roommate or friend that they got their period at the same time. And often, the idea that being so close to someone impacts your chemistry feels good to believe in. But is it true? Can periods really sync up?

What is period syncing?

"Period syncing refers to the belief that [people] who live together or spend a lot of time together will begin menstruating at the same date," said Susan Milstein, Ph.D., sexuality educator on the medical review board of Women's Health Interactive.

You may also know period syncing as the "McClintock effect." That's because the idea of period syncing originated in 1971, when researcher Martha McClintock published the first study examining the phenomenon. This study sampled 135 cis women living together in dorms and concluded the women's periods were, in fact, syncing up. However, this study has been widely criticized because it had a small sample size and didn't control for several variables.

So, is period syncing real?

Well, it depends on who you ask.

If you ask someone with a period, chances are they'll tell you it's real.

Many people who menstruate have a story about someone they spent so much time around that they started their cycles together. And if they don't have a personal story, they probably have a friend with a story.

If you ask the research, it says otherwise.

"Recent data doesn't support the theory," said Bat Sheva Marcus, Ph.D., MPH, LCSW, clinical director of Maze Women's Health.

A study published in 2006 by the journal of Human Nature sampled 186 Chinese cis women living together in dorms and found that "women living in groups did not synchronize their cycles."

Another study, done in 2017 by Clue, a period tracking app, sampled 720 people and concluded that "cycles between pairs and cohabiting individuals did not align." In fact, their evidence "indicated that cycles are actually more likely to diverge, rather than sync, over time."

If you ask medical professionals, their answers vary.

"Period syncing is a myth. There is no biological reason this could happen, and it has been studied and not been shown to occur," said Jen Gunter, M.D., OB-GYN, author of "The Vagina Bible." Gunter, who has a large following online, even made a TikTok about this very subject.

When we talked to Sophia Yen, M.D., MPH, co-founder and CEO of Pandia Health, she agreed: Research says period syncing isn't real. However, Yen did share that she and her friends had experienced what seemed to be period syncing.

And other medical professionals had similar responses: "As an OB-GYN, I hear about female roommates, sisters and mothers getting their periods together all at the same time. Even though medical studies don't believe in this phenomenon, I do believe period syncing exists since I hear about it often from my patients," said Sherry Ross, M.D., women's health expert and author of "she-ology."

If research says period syncing isn't real, why does it feel real to so many people?

People 'sync up' by chance

"An average period is five days every 28 [days], [so] there is always an approximately 20 percent chance that two people who can have periods will be overlapping with their bleeding," and this overlap is often mistaken as synchronization, explained Gunter.

There are other factors that impact periods

"Periods are influenced by many factors, such as hormone levels, stress, drastic changes in body weight or illness," among others, said Milstein. Any one of these factors could impact when someone begins their cycle, and this change could appear as syncing if it happens to line up with a close friend.

Sampling bias

Gunter said the reason it feels like everyone's period syncs up is that "all of the people who have not perceived a connection with other people's periods don't ever speak up." So it may be that "syncing up" is the only story we ever hear and the people who don't "sync up" aren't saying anything.

Scientific research says period syncing is not real. This can be disappointing news for people whose period has come on the same day as a close friend. It's possible that we need more research to really know, and it's also possible that periods just match up by chance or from other factors.

If you ask the research, it says otherwise. There is limited study on period syncing, and a lot of it is outdated. Ross added, "What I have learned during 25-plus years as an OB-GYN physician is [that] not everything that happens to us medically can be proven through scientific research."