Is It True That Women’s Periods Sometimes Sync?

You get your period. Your roommate gets hers, too. Suddenly, you’re comparing calendars and wondering: Did our periods sync?

That theory is an enduring menstrual myth. But despite decades of speculation about women’s cycles shifting and synchronizing when they spend time together, research hasn’t proven it to be true.

“For healthy people living together, proximity doesn’t change cycle timing or frequency,” says Ob/Gyn Stacie Jhaveri, MD. “Periods just don’t work that way.”

So, how did this story start? Let’s find out.

Where did the ‘syncing’ idea come from?

The scientific roots of the period-syncing theory can be traced to 1971, when researcher Martha McClintock studied the menstrual cycles of 135 students at Wellesley College who lived together in a dorm.

McClintock’s findings, published in the journal Nature, suggested that the students’ periods became more closely aligned over time. The phenomenon became known as the “McClintock effect.”

It was an intriguing idea. And some later studies — including relatively recent ones — appeared to find similar results.

But for the most part, researchers haven’t consistently replicated the findings. In fact, most research after McClintock’s study found that menstrual cycles didn’t magically match if women spent time together.

Scientists also raised questions about her methods and statistical analysis.

But the idea stuck around culturally even as the scientific evidence behind it became much less convincing. Surveys show that the vast majority of women believe that menstrual synchrony exists.

Why does it seem like periods sync?

Part of the confusion around period syncing comes from thinking about menstrual cycles as though they run on a fixed monthly schedule. They don’t.

A menstrual cycle is controlled by changing hormone levels in your body, says Dr. Jhaveri. And cycle lengths vary from person to person. Your own cycle can vary from one month to another, too.

That means two people’s periods can naturally move closer together and farther apart over time.

Suppose your cycle lasts about three weeks while your roommate’s lasts closer to five. Eventually, there will be months when your periods overlap. Then, as your different cycle lengths continue, they’ll drift apart again.

“Your cycles will overlap and diverge simply because you have different cycle lengths,” explains Dr. Jhaveri.

And when periods do overlap, it stands out: “You tend to remember the months when you and your roommate have bad cramps at the same time,” she adds. “That’s what sticks in your head.”

Could pheromones affect menstrual cycles?

One proposed explanation for period syncing has centered on pheromones.

Pheromones are chemical signals released by animals that can affect the behavior or biology of other members of the same species. There’s plenty of evidence for their effects among some animals and insects.

Researchers have wondered whether humans could communicate chemically in a similar way — and whether those signals might affect reproductive hormones and menstrual timing.

But so far, there’s no solid evidence that human pheromones cause periods to sync.

“The same effect may exist in humans, but there’s no research to support that claim,” Dr. Jhaveri clarifies.

In other words, pheromones make for an interesting theory. They just don’t give us a scientifically established explanation for why your period and a roommate’s sometimes show up together.

What about the moon?

The moon gets pulled into the period-syncing conversation, too — and it’s easy to understand why. A lunar cycle lasts roughly a month after all. Menstrual cycles occur roughly monthly, too.

Surely, that’s not by chance, right? Not so fast.

“The moon affects ocean tides through its gravitational pull, but the same mechanism does not meaningfully affect the human body,” says Dr. Jhaveri. “There’s no verifiable evidence to date that the moon can affect the ebb and flow of your menstrual cycle.”

Factors that actually affect menstrual cycles

Your best friend probably isn’t changing your cycle, but plenty of other things can, says Dr. Jhaveri. Factors that may affect when — or whether — you get your period include:

• Hormonal birth control: Birth control pills alter hormone levels to prevent pregnancy and can change when (or if) you have a period. Other medications may affect your menstrual cycle, too.
• Extreme or chronic stress: Life’s worries can disrupt hormones that regulate your ovulation and menstruation schedule. The effect can delay your period or bring it on early.
• Chronic illness: A wide-ranging list of health conditions (including diabetes, thyroid disorders and depression) and the medications used to treat them can cause irregular periods.
• Disordered eating: Unhealthy eating patterns can limit the energy and nutrients that your body takes in. If your body doesn’t have enough fuel, it may throw off your cycle and result in delayed or even missed periods.
• Intense exercise: Pushing your body through physical activity can lead to irregular periods. The effect is more pronounced when combined with inadequate nutrition intake.

Remember, too, that some variation in your cycle is normal. Changes don’t always mean there’s an issue.

But if your periods suddenly become significantly different from what’s typical for you — or you’re concerned about missed or irregular periods — talk with a healthcare provider to get answers.

Final thoughts

There’s something kind of fun about thinking you and your bestie, sister or roomie are so in tune that even your uteruses have coordinated their schedules.

But it’s not an example of biological or hormonal harmony.

“Sharing cramps at the same time can definitely make it seem like your periods are suddenly in sync,” says Dr. Jhaveri. “But what you’re really looking at is a calendar coincidence. There’s nothing deeper going on.”