Period Syncing: Does Your Period Really Sync With Close Friends?

Period Syncing: Does Your Period Really Sync With Close Friends?

Have you ever heard of the theory that your period can sync with someone you spend a lot of time with? This "phenomenon" has been talked about for decades, but is there any scientific evidence to support it? Is it real or just a myth?

Let's find out!

Note - we are not talking about cycle syncing, which is a whole different concept.

What do you mean by period syncing?

Period syncing, also known as menstrual synchrony is the idea that women who spend a lot of time together, such as living together or working in close proximity, will eventually have their periods aligned (synced).

This period syncing effect is also known as the McClintock Effect

The theory suggests that pheromones, chemical signals that are released by the body and can influence the behavior of others, are responsible for this synchronization.

This theory was derived from a Nature paper by Martha McClintock in 1971 which gained significant attention for its claim that the menstrual cycles of friends or roommates living in dormitories synchronize gradually.

The findings of Martha McClintock's study, which were derived from monitoring 135 students of Wellesley College, have been under scrutiny for a significant period.

In a 2017 study, which was much smaller in comparison, the concept of period syncing was supported as 44% of the participants who were living with other women experienced period synchrony. The study also found that women who lived together had a higher prevalence of period symptoms such as menstrual migraines. This suggests that women may have an impact on each other's menstrual cycles beyond just the timing of their periods.

Although a few studies have produced comparable outcomes, many of those who replicated the McClintock study believe that there were methodological and statistical inaccuracies in the analysis.

What is the science behind periods syncing

Despite the popularity of the idea, there is little scientific evidence to support the theory of period synchronization.

One study by Zhengwei Yang was done by collecting 186 Chinese women’s data, found that the menstrual cycles of women who lived together did not synchronize any more than the menstrual cycles of women who did not live together.

Another study conducted by the period-tracking app Clue analyzed data from over 7,500 menstrual cycles and found no evidence of synchronization between women who lived together.

Why do women's periods sync up?

Have you ever felt that your menstrual cycle synchronized with that of your close friend or relative, despite being aware that there is no scientific evidence to support the idea of period synchronization?

“I'm not sure if it's true or just a myth, but I gotta say, I totally experienced this thing called period syncing with my roommate back in college.” - Marta UF

While science may not have found a perfect explanation for why a woman's periods may synchronize with that of her relative, roommate, or close friend, there is an alternative explanation - it's just a matter of calculating time and period length.

Confused? Here’s how

If your menstrual cycle typically lasts for three weeks, while your roommates last for five weeks, mathematically speaking, your cycles are expected to coincide at some point.

Why is the period syncing theory hard to prove?

We may not be able to confirm if period syncing is real or not, as there are some uncertainties.

Reason 1

This idea is debated because we are not sure if the chemical signals called pheromones can actually affect when a person's period starts. Pheromones are signals that our bodies produce and they can indicate things like attraction or fertility.

However, we are unsure if one person's pheromones can influence another person's menstrual cycle.

Reason 2

Proving period syncing is also challenging because women have different menstrual cycles. The typical menstrual cycle lasts 28 days, starting with 5 to 7 days of bleeding, but many people don't have this exact cycle.

Some people have cycles that last up to 40 days, while others only have 2 or 3 days of bleeding.

This means that the idea of "period syncing" is not a precise measurement because it depends on how we define the term.

Period syncing might occur mostly due to probability rather than any other reason. For example, if you live with three other women and you have your period for one week every month, it is likely that at least two of you will have your period at the same time.

This makes studying period syncing more difficult because probability plays a role.

Why does the period syncing myth still exist?

Despite the lack of scientific evidence, the idea of period synchronization continues to persist in popular culture.

One reason for this may be confirmation bias, which is the tendency for people to look for information that confirms their existing beliefs.

Additionally, the media often portrays the idea of period synchronization as a fact without providing scientific evidence to support the claim. This can sustain the myth and create a sense of confirmation in people's minds.

So what can really affect your period?

There are several other scientifically proven factors that can affect your period. They are:

Hormonal Imbalances

Hormonal imbalances can be a significant factor in menstrual cycle irregularities. The hormones responsible for your menstrual cycle include estrogen, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

Any imbalances in these hormones can cause irregular periods, heavy bleeding, or missed periods.


Stress is another significant factor that can affect your menstrual cycle. Studies show that high levels of stress can affect the hypothalamus, the gland responsible for regulating your menstrual cycle.

This can lead to irregular periods, missed periods, or even complete cessation of your menstrual cycle.

Weight Changes

Weight changes can also affect your menstrual cycle. Being underweight or overweight can lead to hormonal imbalances, affecting your menstrual cycle's regularity. (study)

In extreme cases, being underweight can lead to amenorrhea, where you stop menstruating altogether.

Thyroid Disorders

Thyroid disorders can affect your menstrual cycle by interfering with the hormonal balance. (study)

Hyperthyroidism and hypothyroidism can cause irregular periods, heavy bleeding, or missed periods.


Certain medications can affect your menstrual cycle.

Hormonal contraceptives can regulate your menstrual cycle, but stopping them can lead to irregular periods. (study)

Other medications, such as anticoagulants and chemotherapy drugs, can cause heavy bleeding or missed periods.


Menopause marks the end of a woman's reproductive years, and it's a natural process that occurs in women over 40.

During menopause, the body undergoes hormonal changes, leading to irregular periods, hot flashes, and other symptoms.


While the idea of period syncing may seem true, the scientific evidence does not support this claim. Women's menstrual cycles are regulated by a complex interplay of hormones and other factors, and synchronization is unlikely to occur simply by spending time together.

It's important to be critical of the information we receive and to seek out reliable sources when evaluating claims like menstrual cycle synchronization.

Frequently asked questions on period syncing

Can periods sync between family members?

There is no scientific evidence to suggest that menstrual cycle synchronization occurs between family members.

Can periods sync between roommates?

While the idea of menstrual cycle synchronization may seem plausible, there is no scientific evidence to support this claim.

What factors influence the length of a menstrual cycle?

Hormonal changes, stress, and certain medical conditions can all influence the length of a woman's menstrual cycle.

Can stress affect menstrual cycles?

Yes, stress can affect menstrual cycles. High levels of stress can cause hormonal changes that can lead to irregular or missed periods.

Is it possible to control the timing of your period?

While it's not possible to completely control the timing of your period, certain hormonal contraceptives can be used to regulate menstrual cycles.

Can period syncing be harmful?

There is no scientific evidence to suggest that period syncing is harmful.

Studies have shown that proximity does not change cycle timing or frequency, and any similarities in menstrual cycles among women living together are likely due to chance. Therefore, it is unlikely that period syncing can be harmful.