Almost every woman can tell the same story. At some point — either in college or when living with a friend — her menstrual cycle synced up with another woman’s.
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It’s a widely known wives’ tale, and many theories suggest that outside factors can push menstrual cycles to shift. But, does scientific evidence exist to back it up? Obstetrician-gynecologist Lynn Simpson, MD, says no.
“For healthy people living together, proximity doesn’t change cycle timing or frequency,” she says. “Periods just don’t work that way.”
A menstrual myth is born
Stemming from a 1971 study published in Nature, the menstrual synchronization myth — called the McClintock Effect — suggests that pheromones or other factors can influence and shift periods for women who live together.
The results, based on observations of 135 Wellesley College students, have long been questioned. While a few studies had similar findings, many who duplicated her study think there were methodological errors as well as statistical errors in the analysis.
More studies, including those most recently conducted, do not support the finding. This includes a 2006 Human Nature study that showed that menstrual cycles don’t align reliably after a year of living together.
What about the pheromone effect?
Pheromones are chemicals released by animals that influence the behavior of others of the same species. Among animals and insects, a lot of evidence exists to support the power of pheromones to impact and change hormonal behaviors.
It’s possible that the same effect exists in humans, but there’s no research to support that claim, Dr. Simpson says.
“Pheromones are largely emitted from the armpits and the groin, but think about the habits of modern-day society,” she says. “Maybe there’s something there, but if you’re washing it all off when you shower, the pheromones aren’t going to have much effect.”
Do the moon’s phases hold sway?
Alongside the McClintock Effect, researchers have investigated whether the phases of the moon can push a woman’s period to start earlier or later in the month.
Although the moon’s pull affects the ocean’s tides, there’s no verifiable evidence to date that the moon can affect the ebb and flow of your menstrual cycle, Dr. Simpson says.
What’s actually happening?
Science may not confirm a chemical or hormonal reason to explain why your period might match up with a roommate or close friend, but there is a mathematical explanation: It’s simply a matter of time. Over time, a woman who has a three-week cycle and another who has a five-week cycle will eventually see their periods coincide and diverge again.
“It’s anecdotal that you’re likely to remember the times you had bad cramps at the same time your roommate did more than the times you didn’t,” Dr. Simpson says. “You will overlap and diverge because women have different cycle lengths.”
If you live with someone for at least a year, your cycles are likely to happen to occur together a few times, she says.
What really can affect your period?
Although science can’t prove whether living with someone or the pull of the moon impact your cycle, there are a few things that can change the timing of your period:
- Birth control pills: The pill alters the levels of progesterone and estrogen in your body to control when — and if — you have a period. That makes it the only outside influence that really influences what your body does. You can manipulate when your period arrives, so it’s essentially a fake period, Dr. Simpson says.
- Extreme stress: Constant worry or anxiety over life or work problems can sometimes delay your period or bring it on early.
- Chronic illness: Living with constant health problems, as well as taking certain medications, can make your period irregular.
- Anorexia/bulimia: Highly restrictive control over caloric intake — either too high or too low — can impact whether you even have a period.
So, although many women may feel an elemental connection with roommates or close friends, it’s probably just coincidence when your periods happen to sync up.