Surprise: Your period probably doesn't sync up to women you hang out with

When you and your roommates are all together on the couch eating chocolate and clutching your hot water bottles, it's easily to buy into the idea that when women spend a lot of time together, their menstrual cycles begin to sync up.. We really want to believe period-syncing exists: 80 percent of women in one study in the Journal of Reproductive and Infant Psychology said it was a real phenomenon, and 70 percent said they liked that it happened, perhaps because it evokes an idea of sisterhood. But is there any truth behind the concept?

In the past, research has said "yes." A 1971 University of Chicago study in Nature found that college friends' periods were closer together at the end of the school year than they were at the beginning. Similarly, a 1999 study in the Journal of Comparative Psychology found that sisters' and friends' periods got closer over time.

But a 1992 meta-analysis in Psychoneuroendocrinology found that the most influential results supporting menstrual synchrony could be explained away by errors, and more recent evidence has been even shakier. A 1993 study in Psychoneuroendocrinology failed to find synchrony in lesbian couples, and a 2006 study in Human Nature once again found no evidence of menstrual synchrony.

RELATED: What Foods to Eat During Each Week of Your Cycle for Less Annoying Periods

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What Foods to Eat During Each Week of Your Cycle for Less Annoying Periods
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What Foods to Eat During Each Week of Your Cycle for Less Annoying Periods

1. The follicular phase: sprouted and fermented foods When: one week after your period

Estrogen is rising during this phase, says Vitti, so the best foods to eat are ones with prebiotics and 3-endole-carbinol, which help your body break down and metabolize the powerful hormone. Some good options are sauerkraut, kimchi, bean sprouts, and broccoli sprouts.

2. The ovulatory phase: raw juices and fresh, whole veggies When: two weeks after your period

Your estrogen levels surge even more this week, so it's key to eat foods that help move it out of your body. Antioxidants and fiber will accomplish this, so load up on fruits and vegetables. To pack a bunch in at once, Vitti recommends this veggie juice recipe. Still, it's crucial to also eat plenty of raw, whole sulfur-rich vegetables (broccoli, bok choy, kale, and cauliflower, to name a few) so your liver gets enough glutathione, a powerhouse molecule with a long list of vital benefits.

3. The luteal phase: greens and grains When: three weeks after your period

At this point, progesterone levels surge along with estrogen, and then both start to wane, which can lead to those dreaded mood swings. That's where vitamin B–rich foods come in: They help your brain produce the pleasure-inducing hormone serotonin. Grains like quinoa and buckwheat are key, while Vitti also recommends leafy greens during this time, since they pack calcium and magnesium that help regulate hormones. As a bonus, both grains and greens contain soluble fiber, which provides additional help with getting extra estrogen out of your body.

4. The menstrual phase: healthy fats and root vegetables When: the week of your period

Your hormone levels go back down during your period, and foods with fatty acids, like salmon and avocado, help keep your mood stable amid this shift. In the meantime, root veggies like sweet potatoes, pumpkin, carrots, and beets provide vitamin A to help your liver process estrogen.

You don't have to restrict other foods from your diet during these stages—just eat more of whatever's optimal. "If you’re craving a burger during ovulation, go ahead and have that with a big salad instead of fries," Vitti suggests. "Listen to your body and honor the phase you’re in."

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Given these mixed findings, the team behind the period-tracking app Clue was curious if they could observe any patterns among their users. Researchers from Clue and the University of Oxford analyzed data from 360 pairs of close women, including friends, siblings, parents and children, partners, roommates, and colleagues.

Not only did cycles fail to sync up; they actually got out of sync. 273 of the 360 pairs (76 percent) had periods further apart at the end of three or more cycles than when the tracking began. Only 79 pairs of women—22 percent— had their periods get closer together. On average, cycles were 10 days apart at the beginning and 38 days apart at the end. Perhaps most surprisingly, cohabiting women were more likely to have diverging cycles, not converging ones. Of the pairs whose cycles diverged, 37 percent were living together, but only 24 percent of those whose cycles converged lived together.

"Our brain likes to look for patterns, so if you have a friend and your friend tells you that they've got their period, if you don't have your period at that particular point, that'll be a piece of information that your brain won't necessarily notice," Clue's data scientist Marija Vlajic told Glamour. "If your friend tells you 'I've got my period today' and you're like 'me too,' you think, 'wow, what's the chance?'" It's the same reason you and an old friend may seem to have a telepathic connection if you think of them and then they email you, when there are plenty of other times you think of them and don't hear from them.

One reason some studies have seemed to support menstrual synchrony is mathematics, Vlajic explains. "Say I get my period today and you get your period in 10 days. At this point, we differ by 10 days, and say that your cycle is two days shorter than mine. So then, when we get our next cycle, the difference will only be eight days."

While Martha McClintock, the author of the original University of Chicago study claiming to prove menstrual synchrony, still thinks there are some situations when it occurs, Vlajic says that's doubtful. "It can be very emotional and very exciting when you notice that you and your friend kind of sync, but it's important to look at it from the more rational side."

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