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Period syncing: is it a real thing?

Laura Harcourt

Written byLaura Harcourt

Dr Shazia Malik

Reviewed byDr Shazia Malik

Happy, friendship and women on a walk at the beach while on a summer vacation
Has your period ever synced up with someone else’s? This is a phenomenon called period syncing. We discuss whether science backs up its existence.
Have you ever noticed when spending a lot of time with someone that your menstrual cycles start to sync up? This phenomenon is known as ‘period syncing’. But is it just an old wives’ tale, or does science back it up?

Many women tell the same story: they were living at home and had their period at the same time as their mother or sister, or they were in halls at university, and their menstrual cycle aligned with someone else’s living in close proximity.

Almost every woman can say their period appears to have synced with whoever they’re hanging out with at some point in their life. So, is it the case that we’re all so in tune with each other that our menstrual cycles shift to occur simultaneously? Or is there another explanation?

What is period syncing?

Period syncing, also known more formally as menstrual synchrony, describes the popular belief that people who spend lots of time together start to menstruate on the same day of each month. An older study in 1999 found that 70% of participants (122 women) had a personal experience of period syncing.1

Period syncing comes from a theory proposed in the 70s called the “McClintock Effect”, that suggests that when women live together, their pheromones can influence each other, causing monthly cycles to shift and line up.2 However, recent research fails to find any scientific basis for the phenomenon.

The McClintock Effect

The McClintock Effect describes a theory that came from research Martha K. McClintock conducted and then published in 1971.2 She studied the menstrual cycle patterns of 135 females aged 17–22 years living in dormitories of an all-women’s college.

Three times during the academic year, McClintock asked each person to note down the date their last two periods began and the two people in their dormitory they saw most often. The data revealed an increase in menstrual synchronisation between roommates and close friends. McClintock questions whether pheromones could play a role in menstrual synchronisation.

Fast forward to 1998, and McClintock releases another study finding a link between compounds from the underarm secretions of women, or pheromones, and the length of the menstrual cycle of women exposed to the extracts.3

Specifically, McClintock’s research found that pheromones from women in the early follicular phase of a menstrual cycle appeared to shorten the cycle of other women. By contrast, exposure to pheromones from women at midcycle during ovulation seemed to lengthen the menstrual cycle of others. Exposure to pheromones after ovulation had no impact on the periods of others in this study. Human pheromones definitely exist, but whether they influence other people's periods is still hotly debated as since then, other studies haven’t been able to see these results and some researchers have implied there may have been some reporting errors in the original study. Others have since debunked this theory too.4

Period syncing with the moon

Given menstruation and the phases of the moon are cyclical, you might wonder if there’s a connection between the two. After all, the term ‘menstruation’ derives from the Latin for month ‘mensis’ and the Greek for moon ‘mene’.5

Scholars and philosophers throughout history have also drawn parallels between the menstrual and lunar cycles, especially since they share a similar cycle length. The menstrual cycle has a median duration of 28 days, while the lunar cycle is around 29.5 days.,6,7

Older research from 1986 discovered a synchronous relationship between the menstrual and lunar cycles in a population of women.8 Among 826 females with normal menstrual cycles, 28.3% of them began their period around the new moon. However, a longer-term 2013 study evaluating menstrual cycles over a calendar year found no such synchrony.9 Recently, the moon and menstrual cycle conversation has reignited. A 2021 study analysed long-term menstrual recordings of 22 women who tracked the start of their period for up to 32 years.10

The study revealed that menstrual cycles longer than 27 days temporarily synchronised with two out of three of the moon’s cycles: the luminance and gravimetric cycles. The luminance cycle had the most influence over the menstrual cycle. The researchers hypothesised that in ancient times, the female menstrual cycle was in sync with the moon, but artificial light and modern lifestyles may have significantly altered this synchrony. Further research with larger study populations is necessary to confirm these findings.

Is period syncing just a myth?

In a word, yes. If you live with several women, the likelihood of some of you starting your period around the same time is relatively high, particularly since regular cycles ranging from 21–35 days are normal.11

However, scientific studies have been unable to prove that menstrual synchrony exists, and when they have, researchers have explained it as coincidental.

A 2006 study collected data from 186 women living in dorms in China.4 The researchers found that women residing in groups didn’t synchronise their menstrual cycles. They also reviewed the first study reporting menstrual synchrony and found that the period synchrony in that study was purely coincidental.
Period tracking apps have been available since 2013, providing researchers with a wealth of data to understand if period syncing is real. In 2020, an estimated 50 million women globally used period tracker apps.12

A period tracking app teamed up with the University of Oxford in 2017 to review data from 1,500 women who suspected they’d experienced period syncing.13 Their data scientists found that the menstrual cycles of individuals living together didn’t align, demonstrating that it’s unlikely that women can disrupt each other’s monthly cycles by being in close proximity.

Other factors that can affect your menstrual cycle

Several other factors could impact the regularity of your menstrual cycle, such as lifestyle factors, medications and health conditions.

Factors contributing to irregular periods may include:14-22
  • perimenopause 
  • primary ovarian insufficiency 
  • losing or gaining a significant amount of weight 
  • eating disorders such as anorexia and bulimia 
  • excessive exercise (especially running excessively) 
  • stress 
  • antidepressants 
  • antipsychotics 
  • steroids 
  • weight loss medications 
  • ADHD medications 
  • hormone contraception, including the contraceptive pill, injections and patches 
  • intrauterine system (IUS) 
  • underactive thyroid (hypothyroidism)
  • overactive thyroid (hyperthyroidism) 
  • miscarriage 
  • fibroids 
  • endometriosis 
  • polycystic ovary syndrome
  • pelvic inflammatory disease
Irregular periods aren’t usually a sign of a medical condition.23 However, if you notice a change to your usual menstrual pattern, your periods last longer than seven days, or you have other accompanying symptoms, consult your GP for advice.

The final say

While period syncing may be an experience reported by many, there’s little scientific evidence to back it up and is likely to be purely coincidental.

Older studies found a link between women living together and period syncing. However, recent research using more extensive data sets reveals that women who live together don’t sync their periods and that earlier research was nothing more than coincidence.

The jury is still out on whether the moon cycle can in any way influence menstruation. We’ll have to wait and see what new studies show. But, for now, if you’re having period irregularities, consult your doctor.

They can rule out medical conditions that could be causing changes to your cycle and suggest the best treatment options.


1. Arden, MA., et al. Menstrual synchrony: Awareness and subjective experiences. Journal of Reproductive and Infant Psychology. 1999;17(3):255–265. https://doi.org/10.1080/02646839908404593
2. McClintock MK. Menstrual synchrony and suppression. Nature. 1971;229(5282):244–245. https://doi.org/10.1038/229244a0
3. Stern K., et al. Regulation of ovulation by human pheromones. Nature. 1998;392(6672):177–9. https://doi.org/10.1038/32408
4. Yang Z., et al. Women do not synchronize their menstrual cycles. Human Nature. 2006;17(4):433–47. https://doi.org/10.1007/s12110-006-1005-z
5. Vidafar P., et al. Light on shedding: A review of sex and menstrual cycle differences in the physiological effects of light in humans. Journal of Biological Rhythms. 2022;38(1):15–33. https://doi.org/10.1177/07487304221126785
6. Thiyagarajan DK., et al. Physiology, menstrual cycle [Internet]. National Library of Medicine. StatPearls Publishing; 2022. Available from: https://www.ncbi.nlm.nih.gov/books/NBK500020
7. Andreatta G., et al. The still dark side of the moon: Molecular mechanisms of lunar-controlled rhythms and clocks. Journal of Molecular Biology. 2020;432(12):3525–46. https://doi.org/10.1016/j.jmb.2020.03.009
8. Law SP. The regulation of menstrual cycle and its relationship to the moon. Acta Obstetricia Et Gynecologica Scandinavica. 1986;65(1):45–8. https://doi.org/10.3109/00016348609158228
9. Ilias I., et al. Do lunar phases influence menstruation? A year-long retrospective study. Endocrine Regulations. 2013;47(3):121–2. https://doi.org/10.4149/endo_2013_03_121
10. Helfrich-Förster C., et al. Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the Moon. Science Advances. 2021;7(5). https://doi.org/10.1126/sciadv.abe1358
11. Grieger JA., et al. Menstrual cycle length and patterns in a global cohort of women using a mobile phone app: Retrospective cohort study. Journal of Medical Internet Research. 2020;22(6):e17109. https://doi.org/10.2196/17109
12. Worsfold L., et al. Period tracker applications: What menstrual cycle information are they giving women? Women’s Health. 2021;17:174550652110499. https://doi.org/10.1177/17455065211049905
13. Clue. Do menstrual cycles sync? Unlikely, finds Clue data [Internet]. 2017 [cited 2024 Feb 19]. Available from: https://helloclue.com/articles/cycle-a-z/do-menstrual-cycles-sync-unlikely-finds-clue-data
14. Peacock K., et al. Menopause [Internet]. National Library of Medicine. StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507826 
15. Ko KM., et al. Association between body weight changes and menstrual irregularity: The Korea National Health and Nutrition Examination Survey 2010 to 2012. Endocrinology and Metabolism. 2017;32(2):248.https://doi.org/10.3803/EnM.2017.32.2.248
16. Bae J., et al. Factors associated with menstrual cycle irregularity and menopause. BMC Women’s Health. 2018;18(1). https://doi.org/10.1186/s12905-018-0528-x
17. Yum SK., et al. The problem of medicating women like the men: Conceptual discussion of menstrual cycle-dependent psychopharmacology. Translational and Clinical Pharmacology. 2019;27(4):127. https://doi.org/10.12793%2Ftcp.2019.27.4.127
18. Jacobson MH., et al. Thyroid hormones and menstrual cycle function in a longitudinal cohort of premenopausal women. Paediatric and Perinatal Epidemiology. 2018;32(3):225–34. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5980701
19. Zimmermann A., et al. Prevalence, symptoms and management of uterine fibroids: An international internet-based survey of 21,746 women. BMC Women’s Health. 2012;12(6):6. https://doi.org/10.1186/1472-6874-12-6
20. Wei M., et al. Length of menstrual cycle and risk of endometriosis. Medicine. 2016;95(9):e2922. https://doi.org/10.1097/md.0000000000002922
21. Rasquin LI., et al. Polycystic ovarian disease [Internet]. National Library of Medicine. StatPearls Publishing; 2022. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459251
22. Jennings LK., et al. Pelvic inflammatory disease (PID) [Internet]. National Library of Medicine. StatPearls Publishing; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499959
23. NHS. Irregular periods [Internet]. [cited 2024 Feb 19]. Available from: https://www.nhs.uk/conditions/irregular-periods
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