The myth of the moon and menstruation

We are often asked to add lunar cycles into Clue, so we ran the numbers to check it out, scientifically. Does the menstruation cycle synchronize with the lunar cycle? Not according to the data.

Our data science team analyzed 7.5 million cycles and found no correlation between the lunar phases and the menstrual cycle or period start date.

“What you normally hear is that you ovulate around the full moon and get your period around the new moon,” said Dr. Marija Vlajic Wheeler. Wheeler is a Data Scientist at Clue who also has a PhD in Astrophysics from the University of Oxford and spent three years as a postdoctoral researcher at the Leibniz Institute for Astrophysics in Potsdam, Germany. “Looking at the data, we saw that period start dates fall randomly throughout the month, regardless of the lunar phase.”

Wheeler looked at period start dates for 1.5 million users of Clue. For their 7.5 million cycles, she found no indication that the period predominantly starts during the new moon, a correlation that is often suggested between lunar and menstrual cycles.

The global average menstrual cycle length is 29 days and the lunar cycle lasts 29.5 days. Statistically speaking, with a random distribution of period start dates, about 1 in 2 women will have their period start ±3 days from either the full or new moon.

This may contribute to creating a false impression that the period does or should start on schedule with the moon for everyone. The lunar and menstrual cycles will correlate more or less for some people, depending how closely their average cycle length matches the lunar cycle, and the variability of their cycle length.

There are different evolutionary theories speculating why the human menstrual cycle length evolved to be so close to the lunar cycle in length. Stories and beliefs connecting the two are also found in various cultures and mythologies. The terms “menstruation” and “menses” even come from Latin and Greek words meaning moon (mene) and month (menses). While the data from 1.5 million users of Clue point to no correlation, many people will still find that observing the lunar cycle in relation to their menstruation has meaning for them personally.