Period Sync? The Science Behind it

If you’re a woman, there’s a possibility that you’ve heard some ladies say that their periods sync with their sisters or friends whenever they are staying together. Is it real, or is it just a myth that coincidentally occurs? We will be tackling this age-old topic in this blog; read on!

What is Period Syncing?

Period syncing is a widespread belief held by most people. When two or more women spend time together or stay together in the same space for a certain period, they begin menstruating on the same day every month.

Another name for it is the “menstrual synchrony” or the “McClintock effect”. This belief is based on the theory that when one person comes in contact with another menstruating person their pheromones- body chemicals influence each other so much so that their menstrual cycle sync with time.

What is most surprising is the fact that most women swear by this phenomenon. However, the medical community doesn’t have any solid or scientific evidence to prove that this is what happens.

The McClintock effect

Period syncing is one topic that has been discussed between mothers and daughters, in girl groups, and even in dorm rooms for ages. In 1971, a researcher, Martha McClintock decided to conduct a study. She analyzed the menstrual data of 135 women living in college dormitories aged 17 to 22.

The study found out that after six months those who have a close relationship with one another had their periods start three to five days apart while the control group had their periods start five to fifteen days apart.

What does science have to say about this?

Some persons took the McClintock effect as true while others in the scientific community debunked the phenomenon as a myth saying that women who ovulate and menstruate at similar times do so by chance and that it’s not as a result of proximity.

In 2012, a study was carried out by the University of California where they compared six years' worth of data on the menstrual cycle of baboons and they concluded- that period syncing happens more by chance than by any other explanation.

Also, another study was conducted by a period tracking app company, Clue, and Oxford University. They analyzed menstrual cycle data from about 1500 people and concluded that women can’t disrupt each other’s menstrual cycle by being close to each other.

A smaller 2017 study tries to keep the idea of period syncing alive with its result of 44% of its participants experiencing period synchrony. Period migraines were also common in women living together. This suggests that women may influence their periods beyond just timing.

Why is period syncing hard to prove?

Period syncing hinges a lot on the premise that pheromones influences when your period starts.

Pheromones are natural chemicals released by our body to other people around us. It usually signifies fertility, sexual arousal, and attraction as well as other things. The idea that it may signal another woman for menstruation to take place is still unimaginable.

Another thing that makes period syncing difficult to prove is the fact that although the average menstrual cycle is 28 days, there are lots of people with varying cycle lengths. Some last as long as 40 days and some have shorter cycles too and are still considered “normal”. So, period syncing appears to be subjective and also depends on our definition of “syncing up”.

In essence, period syncing is more of a probability than anything else. If, for example, you have your period for seven days in a month and live with 3-4 other women, the chances are high that at least two of you will have your period at the same time. This probability is what complicates research in period syncing.


As much as period syncing is proving difficult to prove or disprove, it appears it’ll continue to be an anecdotally proven belief about women’s period. It's normal for your period to sync with a sister, friend, or family member and that may add another layer to your relationship. However, if your period is “out of sync” with the woman you live with, it doesn’t necessarily mean anything is wrong with you.

If you have constant menstrual cycle changes, then Yoni Pearls are just what you need! Still, confused about how to use it and what it does? Check out the depth of helpful titles in our blog section.  

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