Myth: Menstrual Cycle and the Moon

There are many evolutionary theories speculating why the menstrual cycle of women appears to be so close to the lunar cycle in length. Myths connecting the two are also found in various cultures. Even the actual words ‘menstruation’ and ‘menses’ come from the Latin word ‘mensis’ which means ‘month’ and relates to the Greek word ‘mene’ which means ‘moon’.

Menstruation and the Moon Myth

  • According to blood and milk, ancient American indigenous cultures contain moon mythologies about women feeling burdened by the troubles of their families. According to legend, the Raven hears their concerns and calls on the Grandmothers for assistance. In response, “Grandmother Ocean spoke to her sister of the women’s plight. Grandmother Moon responded, ‘I am the power of the feminine. I will send into the women, my sisters, your waterscarrying my power. Once every moon cycle, you shall come into the women through me and purify them.’ And, she did this. Ever since then, every woman has a time each moon cycle when she embodies the power of the moon and her flow is the cleansing of the ocean. We call this the woman’s time of the moon, or moon-time.” The common practice of moon lodges or separated spaces for menstruation honors this time of cleansing and connection. Here, women bled together while the community waited for the feminine insight and wisdom gained during this time of rest. While the name ‘moon cycle’ is still common in modern society, the relationship to the lunar calendar is lost by women who have learned to medicalize the ebbs and flow of their hormones and blood. The increase of neuropeptides, estrogen, and testosterone during ovulation results in more brain activity and creative energy. After ovulation, one might enter a reflective and slowing period as the uterus gets ready to shed its internal lining.
  • Accordind to Science Focus, Charles Darwin thought that the 28-day human menstrual cycle was evidence that our ancestors lived on the seashore and needed to synchronise with the tides. The Moon’s phase certainly has an effect on the behaviour of many animals. Fiddler crabs are more active at full and new Moons because the tides are higher, so their burrows are uncovered for longer. Gerbils avoid foraging at night during a full Moon, because the extra light makes them more likely to be eaten by owls. But the human menstrual cycle is only the same length as the lunar month – it isn’t synchronised with a particular phase.
  • According to Helloclue, 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 who had not tracked the use of any form of hormonal birth control. For their 7.5 million cycles, she found no indication that the period predominantly starts during the new moon, a relationship 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 (though it’s normal to have menstrual cycle lengths between 24 and 38 days). Statistically speaking, assuming that periods start at random times, about 1 in 2 people will have their period start ±3 days from either the full or new moon. This makes sense,  ±3 days from either the full or new moon equals about 14 days of the lunar cycle (the new moon + 6 days, and the full moon + 6 days), which is about half of the lunar cycle. So we would expect about half of all people to have their period during any random half of the lunar cycle.
  • According to one study, synchronicity does exist between menstrual and lunar cycles. The study involved 826 women with a normal menstrual cycle, with participants aged 16–25. A large number of women menstruated during the new moon (28.3%). Conversely, the rate of menstruation on other days was much lower, ranging between 8.5–12.6%.


Many scientists reject the notion that a woman’s 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 women’s periods.