Do menstrual cycles sync? Unlikely, finds Clue data

There’s a wealth of misinformation, myths, beliefs and social stigmas circling around menstrual health. One of these beliefs is the idea of cycles lining up, or “syncing.” For centuries, many people have observed that their cycles match up with friends or family members with whom they spend most of their time — especially if they live together.

But does the data back it up? Because menstrual health has been overlooked for decades by the scientific research community, much about the inner workings of the female body still remains unknown. Our research partner Dr. Alexandra Alvergne from the University of Oxford took a look into the origins of cycle syncing, as well as past research.

To date, no studies or mathematical analyses have proven the existence of cycle syncing. The first cycle syncing study, conducted in 1971, introduced the idea of the “alpha uterus,” a uterus with a strong hormonal pull that influences the cycles around it to ovulate and menstruate in unison. Other theories include “socially mediated synchrony,” which claims that women sync to become sexually receptive at the same time, potentially preventing monopolization from a single dominant male.

Thanks to our global community tracking every day (we just hit 1 billion data points!), we’re able to test these commonly held beliefs with data, so we started an investigation to see if cycles really sync.

We reached out to Clue users and asked if they thought their cycles had been syncing with someone (who also uses Clue). We asked what type of relationship they’re in (friends, siblings, parent/child, partners, roommates or colleagues), if they live together and if they’re on hormonal birth control.

We received over 1,500 responses and narrowed it down to 360 pairs with at least three cycles over a similar time period that we were able to review.

We analyzed a minimum of three consecutive cycles for each of those pairs and found that 273 pairs actually had a larger difference in cycle start dates at the end of the study than at the beginning of the study. Only 79 of the pairs behaved the opposite way, with the gap between cycle start dates narrowing over the course of the study.

In other words, according to these results, cycles are actually more likely to diverge (get out of sync) over time.

For the full sample of 360 pairs, the average difference at the beginning was 10 days and 38 days at the end.

We also noticed that living together did not increase the likelihood of syncing. 37% (100 out of 273) of the pairs with diverging cycle start dates lived together, compared to 24% (19 out of 79) for those with converging cycle start dates.

From our small pilot study of Clue users, our data scientists found that cycles between pairs and cohabiting individuals did not align. Our statistical evidence also indicated that cycles are actually more likely to diverge, rather than sync, over time.

We’d like to continue to do more analysis on this topic, and others in the under-researched field of female health. If there’s a topic or area of study you’d like us to explore, contact us on Twitter @clue.

Details of study

Of the 1,500 people who agreed to participate in the study, about 50% of the pairs had to be eliminated because they didn’t both have Clue accounts, which we needed to analyze the data. From there we eliminated any pairs who hadn’t both had cycles in the past year, were on hormonal birth control and didn’t live in the same city, bringing the study down to 500 pairs. We then narrowed the study down further to 360 pairs with at least three cycles — starting after the pairs knew one another — over a similar time period. We ran a statistical analysis on all 360 pairs to see whether their cycles synced and compared this with a control group of completely random pairs who had no relationship to one another. 273 out of the 360 pairs had a smaller difference in cycle start dates at the beginning of the study than at the end — within this group 100 of the pairs lived together. Only 79 of the 273 pairs behaved the opposite way, with the gap between cycle start dates narrowing over the course of the study — and within this group 19 of the pairs lived together.