Perhaps because the menstrual and lunar cycles are similar in length, many early civilizations believed that the moon determined when women could become pregnant. This could explain why female moon deities—from the Chinese goddess Chang’e to Mama Quilla of the Incas—figure so prominently in mythologies from around the world. In the 1950s, the Czech doctor Eugene Jonas stumbled across an ancient Assyrian astrological text stating that women are fertile during certain phases of the moon.
He based an entire family planning method on this hypothesis, telling his patients they ovulated when the moon was in the same position as when they were born. According to another theory that persists to this day, full moons cause an uptick in births, flooding maternity wards with mothers-to-be in labor. Recent studies have turned up little statistical evidence for moon-induced baby booms, however, and most experts think any lunar effect on procreation is imagined.