Is It True that Women's Periods Synchronize?

It's a popular belief that women's periods synchronize when they live together. While it's true that a 50-year-old study affirmed this belief, there's still no definitive proof. Find out more.
Is It True that Women's Periods Synchronize?
Karina Valeria Atchian

Written and verified by the doctor Karina Valeria Atchian.

Last update: 25 August, 2022

The idea that women’s periods synchronize is a popular myth that people have been citing for the last fifty years. Psychologist Martha McClintock first introduced this idea after a study she conducted with women living together in college dorms. Her research was even approved by the scientific journal Nature .

Her theory of menstrual synchrony states that when women live together or spend a lot of time together, their menstrual cycles begin to coincide. According to McClintock, her theory was confirmed. Later, it was said that pheromones were responsible for said synchronization.

What are pheromones?

Pheromones are a type of ectohormone that scientists have studied in both rats and other animals. They play an important role in communication and behavior among members of the same species.

In the case of human beings, studies haven’t demonstrated communication via pheromones in strict terms. Therefore, we can’t conclude that this hormonal mechanism explains the theory that women’s periods synchronize. Clearly, more studies are necessary in order to confirm or negate the theory of menstrual synchrony.

How does a woman’s menstrual cycle regulate?

A woman's biological clock.

Currently, there’s a great deal of knowledge about the female physique. It’s a subject that science has studied deeply and continues to do so.

From the time a woman is born, she has ovules in her ovaries. These remain there, unmodified, until the beginning of puberty. Then, thanks to the changes that take place in this stage, women enter their fertile stage. From this point on, women experience menstruation each month.

The regulation of these cycles occurs through the effect of stimulators and inhibitors on the part of hormones in the brain and ovaries. An area of the brain called the hypothalamus periodically releases the hypothalamic hormones involved (LH y FSH). These hormones have a direct impact on the ovaries.

As a result of this hormonal secretion, the ovary responds by releasing a mature ovule. At the same time, it releases steroid hormones that are different from those that the hypothalamus releases. The mature ovule is ready for fertilization. And, if sperm in good condition reaches the ovule, then it will become a zygote.

During this stage, a thickening of the endometrium–the internal layer of the uterus–also occurs. If fertilization of the ovule takes place, it will implant in the thickened endometrium several days later. However, if there’s no fertilization, then the endometrium will undergo certain changes that will lead to its detachment.

The entire process described above will repeat every 28 to 35 days, in general. The first day of a woman’s period is considered day 1. And, from that point on, women can count in order to determine their most fertile days, for example.

What do we know about the variability between periods?

In general, menstrual cycles have little variability between the ages of twenty and forty. In other words, this is the stage of the greatest regularity in regard to dates and monthly repetition of the signs of menstruation.

Prior to the age of twenty, and after the age of forty, women experience the greatest variation among successive cycles. This means that irregular periods are more frequent. Various situations can produce variability in a woman’s cycle, besides age itself.

During adolescence, irregularity is common because a woman’s hormonal system has yet to reach full maturity. This same variability occurs in women over the age of forty and until they reach menopause. However, in this case, it’s because their ovarian reserves are running out.

What’s more, stress, obesity, low weight, thyroid problems, medication, and certain illnesses like diabetes can also produce irregularities. The changes can be occasional and pertain to one cycle in particular. However, they can also last for several months.

Keeping track of your period.

See also: The Symptoms of Menopause

Do women’s periods synchronize? The evidence is inconclusive

In 2006, Anna Ziomkiewicz, associate professor at Jagiellonian University, concluded that she did not observe synchronization between the woman that participated in a study similar to McClintock’s. We can say, then, that there’s still a lack of conclusive evidence of the existence of menstrual synchrony, or of pheromones in humans.

Many defend the belief that woman’s periods synchronize. However, if we take a hard look at the current facts, we can only say that the claim is a myth. Perhaps, in the future, studies will manage to provide more insight and proof.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Corrine K Welt, MD., retrieved on 14 May 2020, Physiology of the normal menstrual cycle, Evidence-based Clinical Decision Support- UpToDate.
  • B. David Vantman and col., Reproductive physiology and evolutive changes with women age, Tema central: Infertilidad, Vol. 21. Núm. 3, páginas 348-362 (Mayo 2010).
  • Precone V and cols, Pheromone receptors and their putative ligands: possible role in humans, Eur Rev Med Pharmacol Sci. 2020 Feb;24(4):2140-2150. doi: 10.26355/eurrev_202002_20394.

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.