Can our periods really sync up?

We’ve all heard it; hang around a pack of gals for too long, and you’ll soon be on the 'same cycle'. But can our periods really sync, or is it a myth?

Those who remember the anonymous questions box in sex ed class will probably also recall that it was a sess pool of ridiculous biology inquiries and wives tales garnished with the odd secretly helpful question and inappropriate joke.

But guaranteed, every sex-ed teacher would have unfolded a note asking the same question: do women’s periods really sync up?... Bonus points if you also believed that your period would align with an ‘alpha female.’

Where did the period syncing myth come from?

Mothers have been telling their daughters for centuries; if you hang around with another woman for too long, you’ll eventually end up on the same ‘cycle’. Meaning your periods will become synchronised.

Syncing periods have long been the topic of women’s locker room conversations. But it became a matter of real scientific debate back in 1971 when researcher Martha McClintock conducted a study of 135 women sharing a college dorm to see if their periods aligned.

McClintock didn’t track ovulation or the participants’ menstrual cycles as a whole, but she did log when they each started and finished their monthly bleeding. By the end of the study, McClintock concluded that the women’s periods had aligned. After her discovery, period syncing became a phenomenon known as ‘the McClintock effect.’

Period syncing - another menstrual myth

Unfortunately for Martha McClintock, modern research has delved deeper into the complexities of the female menstrual cycle. Many studies - coupled with the popularity of period tracking apps - show that synced periods just aren’t likely.

In fact, Beverly Strassman, professor of anthropology at the University of Michigan, used McClintock’s original conclusion as the basis for a similar study in 1999 and found synchronised periods to be nothing more than a mathematical (and hormonal) coincidence:

If the menstrual cycle is 28 days long, then 14 days apart would be maximal asynchrony. By chance alone, one would expect two women to be seven days apart (half of 14 days). Given that menstruation can last five days, overlapping periods are a common occurrence. That women synchronise to each other, however, is a myth.

Perhaps the most significant blow to synchronised periods was delivered in the form of a 1,500 participant strong pilot study conducted by the period tracking app Clue alongside Oxford University. The study asked participants if they believed their period had synced up to someone else’s as well as their relationship to that person, if they lived together and if they were on hormonal birth control. Researchers then narrowed it down to 360 pairs of period pals and tracked their cycles over three consecutive months.

You can probably guess by now that there was no evidence to support synchronised periods. But, what you might be surprised to know was that the study found that for 76% of participants, their periods actually got more out of sync than closer together.

Can you get pregnant while on your period? Can you get pregnant while on your period?