A full lunar cycle lasts from one new moon to the next and takes 29.5 days. The average menstrual cycle is also around 28–29 days long. However, the actual duration varies from person to person and month to month, ranging from approximately 21 to 35 days.
According to one study, synchronicity does exist between menstrual and lunar cycles. The study involved 826 women between 16 and 25 years old, with normal menstrual cycles. A large number of participants menstruated during the new moon (28.3 percent). Conversely, the rate of menstruation on other days was much lower, ranging between 8.5 and 12.6 percent.
In a recent study, participants with menstrual cycles longer than 27 days tracked their cycle synchronicity for around 14 years. The study showed that their menstrual cycles were indeed sometimes synchronous with the moon’s cycles. However, with age and exposure to artificial light, menstrual cycles shortened and the synchronicity was lost.
Some people believe that in ancient times, people’s reproductive behavior was synchronous with the moon, but with modern lifestyles, reproductive physiology and behavior have changed.
In the 1950s, Dr. Eugene Jonas looked for a scientific explanation for questions regarding female fertility. He noticed that a number of people became pregnant during their period or when they weren’t supposed to be ovulating.
Dr. Jonas believed this pointed to multiple fertile periods — one based on the body’s menstrual cycle and the other based on a lunar cycle that influenced their fertility. The doctor, who believed in astrology and had other unorthodox theories, was convinced that the moon impacted female fertility. However, no scientific evidence has supported his theory.
There’s no scientific evidence that having a cycle that syncs with the moon is of any benefit, but folklore and alternative health advocates say there are benefits to menstruating during the new moon and ovulating during the full moon.
The main way menstrual and lunar cycles are related is the duration of each, which carried special significance for early humans.
However, this correlation isn’t supported by science. Although similar in length, the menstrual and lunar cycles aren’t related.
So why do so many cultures link the menstrual cycle and the moon? People have made lunar calendars since the lunar phases were first charted. Many cultures based their religious ceremonies around the lunar cycle.
For example, some Native American tribes tracked seasonal events using the lunar calendar. They called the moon different names based on the season. The Corn Planting Moon referred to May, and the Travel Moon referred to beaver trapping in autumn. The practice isn’t dead; some cultures use lunar calendars to note fasting and feast days.
Menstrual cycles roughly follow the same timeline, so it’s natural to try to use the moon’s cycles to track menstruation or ovulation. However, this theory cannot be applied to everyone, and even the most regular cycle can vary occasionally. Attempts to tie the moon to female fertility or contraception haven’t proven universally effective.
This belief may have come from a desire to be connected to the flow of the universe. It can be comforting to look at the night sky and remember that hundreds of cultures and billions of people have seen the same stars and moon.
Although several cultures have some tradition that connects menstrual cycles and the moon, studies suggest otherwise. Many scientists reject the notion that a period coincides with moon phases. Instead, the similarities seem to begin and end with the regular 28-day average cycle for both the moon and periods.