Why do Periods Sync Up? Everything You Should Know

Last updated 02.22.2024 | by Sabrina Johnson | 7 Minutes Read

This article has been reviewed and fact checked by Dr. Karen Pike, a senior physician administrator and board-certified emergency room doctor actively working in northern California. Read more at our medical disclaimer page.

Have you heard of period syncing? Do you think it is real or considered the same as a myth? If you aren’t unaware of the same, let me explain it briefly before going into the depths. There is a belief or a fact doing the rounds that women living together had their periods synced up with each other. Is it just an old wives’ tale, or are there any concrete facts supporting this menstrual synchrony?

Here is a finding that I recently went through, which I would also like to share with you all. The motive was to reexamine that women who live together experience their menses simultaneously. Firstly, data regarding the menstrual cycle of 186 Chinese women were collected. The women mentioned above mostly lived together in dorms. As per the study, it was seen that women residing in groups did not have their periods simultaneously. I heard a friend mention that three of the five roomies she stayed with in high school had their periods start at the same time. Is this sheer coincidence, or do we have some concrete facts to support the statement?

We will eventually get to know the same throughout the article. I have discussed what period syncing is and even mentioned the McClintock effect, one of the main theories. I have also spoken of some essential studies to validate the authenticity of period syncing. Read on to know more.

Period Syncing – Understanding the Concept

Taking a cue from what I mentioned above, period syncing, also known by other names like the McClintock effect and menstrual synchrony, is the theory or belief that your period syncs up with someone you acquaint or live with every day. To explain in detail, when you are close to another menstruating woman, your pheromones are influenced to the extent that you experience your menses almost at the same time as the other. It is mainly reported in all-female groups living together, like a family of mothers with their daughters.

Pheromones are chemical signals that our body releases through urine, sweat, breast milk, semen, and vaginal discharge. Pheromones play a significant role in initiating communication between humans and animals. Many even believe in the alpha female’s role in determining the period cycles. Let us look into it further.

The McClintock Effect

The discussion of menstrual synchrony remains incomplete without the McClintock effect. Martha McClintock published a study in 1971 regarding menstrual synchrony, which made many in the scientific community consider the seriousness of this aspect. She conducted a study on 135 college women between 17 and 22 years old, living in dorms in a women’s college situated in the suburbs.

The study aimed to determine if menstruation in these women occurred at the same time or not. The study only highlighted the duration of the females’ monthly bleeding. It didn’t analyze other factors like the ovulation time of women. The final results of the study indicate menstrual synchrony. This hasn’t been the only study in this regard. Several other research studies have also been conducted that point to the statistical and methodological errors of McClintock’s analysis. Let’s see what other studies have to say regarding period syncing.

Results of Other Research Analysis on Menstrual Syncing in a Woman

Results of Other Research Analysis on Menstrual Syncing in a Woman

Over the decades, several other studies have been conducted to assess the authenticity of menstrual synchrony. Many of the recent research has considered this nothing more than a myth and debunked the same. The first study that I already mentioned at the beginning was conducted in 2006 on 186 women co-living in dorms. The results negated menstrual synchrony and mentioned that women living together never experienced periods at the same time. If they did, it was nothing more than a coincidence.

The next study I would like to touch upon is the one that was collaborated by Oxford University and the period-tracking app Clue. Around 360 pairs of women who used this app were surveyed. The groups included friends, roommates, mother-daughter, and friends. The results didn’t find any menstrual syncing.

Another study, a smaller one though, was conducted in 2017 on university students between 18 and 30 years of age. This one is a little different as it mentions the prevalence of menstrual migraine among women living together.

So, most of the studies conducted of late do not validate the occurrence of menstruation at the same time in women living together.

Is Menstrual Syncing Related to the Moon?

Is Menstrual Syncing Related to the Moon?

Did you know? Menstruation comes from “mene,” the Greek and Latin word for moon. Many believed that the lunar cycle played a significant role in determining a woman’s fertility rhythm. A few studies even reveal how your periods sync with the lunar phase.

A 1986 study that collected data from 826 female participants concluded that around 28.3% of them experienced their periods during the new moon. The study mentioned above encompassed a small population, and if we were to take the results, we could deduce that one in four women had their periods as per the lunar cycle. In 2013, a separate study deduced that there isn’t any relationship between a woman’s menses and the moon.

The latest finding in this regard was a 2016 study on people who menstruate. It suggested the connection between the moon cycle and a woman’s menstruation is a myth. Around 7.5 million menses were analyzed through this research. The study concluded that there isn’t any synchronization between the lunar cycle and a woman’s menses.

Role of Mathematics in Menstrual Synchrony

Menstrual synchrony has been regarded as a myth in many studies. However, this is often justified through a mathematical explanation. If two women have a 3-week and 5-week menstrual cycle, respectively, then there is a chance that they will experience an overlap in their periods. Have you ever noticed that both you and your roommate had cramps more or less at the same time? That is because the cycle lengths of women vary. So, if you’ve lived with someone for almost a year, your cycles may likely sync.

Why is it Difficult to Prove Menstrual Synchrony?

Period syncing seems challenging to establish due to a lack of concrete proof to support this fact. This aspect is controversial because research hasn’t been able to show the role of pheromones in stimulating menstruation in women living together.

Moreover, no two women have the same menstrual health. The period symptoms also vary. If you live with other women, you could have a shorter cycle of two to three days in a specific month. In contrast, your roomie has a longer cycle of seven days. In such cases, period syncing may not be possible.


What are the factors that affect or have an impact on your period?

Some common factors that influence your menses include birth control pills and even any chronic illness that may cause irregular periods. Chronic anxiety may also cause your periods to occur early or late. Inadequate exercise and increased calorie intake are the other factors that influence your periods.

 Is menstrual synchrony observed between mother and daughter?

A study conducted in 1993 showed that if a mother and daughter lived within the same domicile, a significant amount of synchrony could be noticed in them. However, whether it is real or myth is debatable, as many studies have shown evidence against this notion.


The discussion we have had so far highlights the fact that proving or disproving menstrual synchrony is a mammoth task. It’s a natural human tendency to link our physical and emotional experiences. It sometimes feels good to think that the bond between you and your close ones is so strong that your menses also happen close by.

However, when women living in groups do not experience a sync in their cycles, it doesn’t mean there’s something wrong with their relationship or periods. Though several studies have mentioned that there is barely any truth to period syncing, many women religiously believe in the same. In an excerpt, I came across on a BBC news website, a woman humorously revealed what a team player her womb was. She would menstruate whenever she spent some time with another woman who was having her periods then. She even mentioned her friend, whom she thought had an alpha womb, as anyone in proximity to her would also have their menses. So, to end our discussion here, menstrual synchrony isn’t proven, and for the time being, it will continue to exist as a proven belief.


  • Sabrina Johnson

    Meet Sabrina Johnson, a compassionate author and a seasoned expert in Obstetrics and Gynecology. She is a driving force behind Simply Menopause, where her extensive medical knowledge and empathetic nature come together to empower women in their menopausal journey. Sabrina offers culturally sensitive guidance and support through her approachable writing, making her a trusted friend on the path to menopause wellness.

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