If you’ve ever lived in a household that is predominantly made up of women, you may have noticed or heard of a strange phenomenon where everyone’s menstrual cycles seem to sync up. Was it just coincidence, or is there a biological element behind it? An expert from the Texas A&M College of Medicine talks about the science behind syncing cycles.

What’s the verdict?

“The short answer is we don’t know,” said William Price, MD, director of obstetrics and gynecology at the Texas A&M College of Medicine. “There hasn’t been a shortage of research done about it, but everything has turned up inconclusive.”

The research started in the 1970s when a psychologist from the University of Chicago asked 135 college girls living in dorms to recall their menstrual cycle start dates at three times throughout the academic year. She noted that females who were physically closer in proximity to each other (in this case, living in an all-female dorm) have cycles that start significantly closer than they did previously.

However, ever since then, numerous research and studies have been done, and the results remain inconclusive. The disagreement isn’t always about the results per se, but on the interpretation of the data. For example, in 1992, one researcher found errors in the initial University of Chicago study that “increase the probability of finding menstrual synchrony in the sample,” and stated that neither that study nor any other demonstrated menstrual synchrony.

“It seems that for every study that said that cycles can sync, there has been another that disagrees,” Price said. “There have been some theories that it happens in animals, like new world monkeys and rats, but even then, there is still a lot of debate.”

A possible theory

Many people who believe the synchronization attribute it to pheromones, a secreted chemical factor that triggers a biological response in others. Although it may seem that even if it does exist, any syncing would serve little purpose, our hunter-gatherer ancestors could have benefited from this pheromone-phenomenon.

“It makes sense as a survivalist theory,” Price said. “If all of the men of a tribe were out on a long hunt and only came back for brief periods of time, then it might be beneficial for the females to have their menstrual cycles synchronized.”

This theory could explain the benefits of having the female biological clocks all running in sync, but is there any evidence?

Coincidence—and intentional regulation

Because cycles and periods are fairly regular, they become a numbers game.

“There’s a narrow window that exists, and some women may just happen to fall in that area at the same time,” Price said. “A cycle is about 28 days, and a period is about five days, so there’s about 25 percent of the adult female population that may be synchronized—just by coincidence.”

We don’t know if the female body naturally syncs its cycles with women nearby, but it is possible to manage and regulate it. “Birth control hormones can help keep female cycles more regular and predictable,” Price said. “We also have medication called human chorionic gonadotropin (HCG) that can induce ovulation. It’s used in in vitro fertilization and requires a pre-ovulatory visit with your physician for that to work.”

Whether it’s because of biology or statistics, the debate over whether cycles can sync isn’t going away anytime soon. If you think that it’s happened in your house or among your friends, then it very well may have, but according to science, it was probably just a random occurrence.

— Dominic Hernandez

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