Cycle Syncing: Is It A Real Thing + Why Does It Happen?

Here at mbg, we talk about periods quite a bit: cool period accessories, whether it's safe to do yoga on your period, and even cover how to hack your menstrual cycle for a happier life. To say we're grateful that periods are having a moment would be a massive understatement. What a time to be alive! We never imagined we'd be adopting the notion that periods are more of a opportunity than an obstacle, but here we are.

When my period started syncing to the women around me, I got curious. It's happened several times: first, when I lived with female roommates and now at work. In my experience, the woman on birth control would lead the syncing. Now that I'm off the pill, I'm the one whose period is constantly adjusting and readjusting to my colleagues'.

This phenomenon is called The McClintock Effect, as Martha McClintock was one of the first to documented scientific evidence of menstrual synchrony. Following her 1971 paper, other researchers found flaws with her methodology, and recent research is conflicting at best. How unsatisfying, right?

As an OBGYN, I hear about female roommates, sisters and mothers getting their periods together all the time. So, I do believe menstrual synchrony exists since I hear about it often. —Dr. Sherry A. Ross, OBGYN, author, and women's health expert

Based on testimonials of my patients and personal experiences, I do believe in menstrual synchrony. Indeed, the research is conflicting, however, that is the case for almost anything being researched today. —Dr. Prudence Hall, functional OBGYN and Founder of The Hall Center

This so rarely happens I think because everyone is on their own schedule with sleeping and waking that it's less common. In addition, now versus 1971 when McClintock did the study, the endocrine disruptive chemical exposure today eclipses even the best imagination of anyone from that time, and women are struggling in general just to have a normal functional cycle, let alone have the possibility to sync with friends. —Alisa Vitti, integrative nutritionist and hormone expert

I absolutely believe in menstrual synchrony. I have received feedback from the women who attend the monthly moon circles on a regular basis that they begin to all link up on either the new or full moon cycle. —Paula Mallis, doula and spiritual counselor

I really want to believe in the idea of menstrual synchronicity. I'd like to think that it is completely possible. The thought of syncing up with the women around me sounds pretty damn powerful. It would create strong feelings of empathy and connectedness. I know that there is no real solid scientific evidence that backs this up other than my/our experiences. — Lindsey Bliss, doula and mother of seven

I grew up with Martha McClintock. Literally around the corner from us my whole childhood. In Concord MA. She was one of the "older kids" but always scary smart. I do believe in the effect even though I know that it's currently controversial. —Alice Domar, health psychologist and women's health expert

The first paper published in 1971 showed menstrual synchrony existed. This paper showed when women spend more than 3 months together their periods will start within 4 days of each other’s. This study has not been duplicated through controlled medical studies to support that this actually is a real phenomenon. It could be an old wive’s tale but even with those there is some notion of truth.

What I have learned during the 25 years I have been a physician is not everything that happens to us medically can be proven through scientific research. —Dr. Sherry Ross

Although we have not been able to measure pheromones and the existence of them is unproven, there is little doubt they exist. Pheromones are considered to be airborne chemical signaling molecules that are released by humans, animals, and plants into the environment and they can affect the physiology and or behavior of other members of the same species.

In particular, the research on the phenomenon of menstrual synchrony was shown to be robust in for mothers and daughters as well as roommates.

I think it may be difficult to draw conclusions from studies because there are factors that cannot be measured that play a role in menstrual synchrony such as the particular nature of the relationship and the specific living arrangements of the women. —Dr. Prudence Hall

I do think the strongest piece of the [McClintock] study is that women's cycles can be affected by lunar phases in the same location in the same way and that if they are sleeping and rising at the same time, therefore exposed to the same light/dark exposure, that those two things can create synchrony via the pineal gland, which consequently moderates melatonin's effect on ovulation patterns. —Alisa Vitti

One theory I’ve heard is that it’s not the women’s pheromones that they’re responding to… but the pheromones of a nearby male. And if they all are in close proximity to the same male, perhaps that would cause them to sync to each other. Interesting theory. Kind of annoying to me - I’d rather us women sync up with each other than have some man dictating us all :) But I haven’t seen any studies to confirm this theory either. —Katherine Altneu, acupuncturist, herbalist and fertility specialist

I have studied ancient women's rituals. Women used to coexist and live together in ways that we don't today. In those times it was incredibly common for women to retreat to what they would call the "Red Tent." This ritualistic retreat was a sacred way to honor the menstrual cycle. —Paula Mallis

I was chatting with my business partner at Carriage House Birth and she shared that when she took her doula training (all women) they all seemed to sync up. The instructor of the training had gone without a moon for many years and miraculously started menstruating with the group. Amazing right? —Lindsey Bliss

I have experienced this myself with both my daughter and great friends. —Dr. Prudence Hall

No, I haven't had any true experiences of synchrony. But keep in mind that I haven't lived with women since college, at the height of my PCOS so I wasn't cycling at all. I'm a great example of how endocrine disruption trumps synchrony. —Alisa Vitti

When I was living in the Delta Gamma Sorority house in college my 4 roommates and I started getting our periods during the same time of the month—go figure! —Dr. Sherry Ross

I certainly have experienced this effect personally many times in my life. But who knows if it’s a coincidence or not. I’ve not seen many studies confirming it. —Katherine Altneu

Since I began to lead women's moon circles my cycle has linked up with other women and the full moon cycle. —Paula Mallis

Growing up I remember having my period when my sister and my mother had theirs. It seemed like a more than just a regular occurrence. When my step-daughter started menstruating I noticed that our cycles synced up too because my tampons would always go missing whenever I needed them. —Lindsey Bliss

Before I went on the pill, my menstrual cycle was synchronized with my best friend at high school, who I spent all of my time with. Once I went on the pill though, my ‘period’ (not actually a true period, but a hormone withdrawal-bleed) was determined by synthetic hormones, rather than other factors, and our cycles fell out of sync. Since coming off the pill though I haven’t really had a clear experience of menstrual synchrony, as I don’t live with any women and spend most of my time with my boyfriend. There’s been a few coincidences, but there’s no one group of women I spend all of my time with. —Jodie Milton