It’s that old statistician’s lament: correlation isn’t causation. Two things that seem like they sync up often turn out to be a coincidence. Take, for example, a woman’s menstrual cycle and the lunar calendar. At first glance, they seem like they might somehow intertwine: after all, they both last about a month, they both have distinct cyclical phases, and the moon plays a key role in other earthly liquid phenomena like the tides. But does the science check out?
Spoiler alert: nope! Not even a little.
First, some backstory. Humans have a long history of seeing the moon as a feminine force: in ancient Greece, Phoebe was the lunar goddess (sister of Helios, the sun god, naturally). Her other name? Mene, which is one of the roots of the words menstruation. The Mayan moon goddess is associated with fertility and procreation, but also with crops and water: basically, anything that grows from the earth. And Artume, an Etruscan moon goddess, was linked to the night and fertility. These mythologies tend to depict lunar deities as smart, beautiful, and governing natural cycles where fertility would be especially important, like farming and the annual harvest.
On the dark side, werewolves and vampires are both linked to moonlight, and while those figures are fictional, their interest in blood just can’t be denied. Werewolves are an especially potent character, since they’re “normal” for 28 days and a monster for a single full-mooned night. There’s a hoary old joke about ladies on their periods in that one, right? (Sigh.)
Let’s get out of the Turner Classic Monster stable and back into the science lab. Some facts: the moon’s cycle is highly regular, with a fresh new moon debuting every 29.53 days. This happens regardless of what else is going on in the universe. Solar flares, space landings and asteroids don’t affect the moon. Our little satellite spins dependably along, fattening itself up for the full moon and then waning into a sliver before hiding itself entirely as the new moon.
Women’s cycles are a little more…unpredictable. Our systems are a complex biological loop that can be affected by stress, hormones, sickness, and even exercise. It’s common for people’s cycles to vary between 21 days and 35 days. Even women who report regular periods can have an unpredictably long or short cycle once in a while. Here’s why: each month, the body tries to release an egg at the best time to achieve pregnancy. If the body senses a fever or stress hormones, there’s no use releasing an egg into a body that’s too hot, stressed, or hungry to keep it healthy. Our bodies are willing to wait a little while to keep the egg optimized and ready for fertilization, and that’s when we’re faced with a cycle that’s longer than usual.
So why do our cultures link the moon and the menstrual cycle?
Well, for starters: it’s easy. Humans have been creating lunar calendars since we first noticed the moon waxing and waning, and some religions and cultures still use them to this day. Some Native American tribes used full moons to track important seasonal milestones—the Corn Planting moon in May, or the Travel Moon in the fall beaver-trapping season, for example—and both the Islamic and Jewish faiths still use a lunar calendar to establish important feast and fasting days throughout each year.
Menstrual cycles seem to fall along roughly the same timelines, and it can be tempting to use the moon’s cycles to track an ovulatory or menstrual cycle. Tempting, but unless you’re ready to pick out baby names, you might want to rethink it. Studies that link the moon with fertility or contraception have been inconclusive, and for a good reason: every woman is different. Every cycle can be different.
It can also be beautiful to consider yourself part of the fabric of the natural world. It can be empowering to remember that cultures throughout the world and history have looked up into the night sky and seen a powerful and unabashedly female figure shining down on them. The moon’s waxing phase, as it fills out and brightens the night sky, is an especially vivid symbol of pregnancy and fecundity, which are, of course, directly related to menstruation.
But relying too heavily on the supposed link between menstruation and the moon can be a recipe for heartbreak, both for people trying to achieve pregnancy and those who are actively avoiding it. Instead, turn your gaze inward and, uh, slightly downward: indicators like cervical mucus, basal body temperature and ovulation cramps can be more reliable signs of how your menstrual cycle is progressing.
No matter how you see the moon — whether with scientific interest, or with wonder, or both — it’s hard to deny that its cycles are powerful. But so, too, are women’s menstrual cycles. Just think twice before seeing that correlation of power as one cycle causing the other.
Kaitlyn Kochany is a Toronto-based writer, blogger, sex nerd, cyclist, amateur yogini and co-op housing enthusiast. She can be found at @terrorofthe416 and at http://kaitlynkochany.com.