why link menstruation & the moon

Timing

The two cycles last for roughly the same amount of time.

Action

The moon visibly waxes and wanes, apparently grows larger and then smaller again. A woman's body goes through the same process. The uterine lining grows heavy with blood, then sheds the build-up if conception has not occurred.

Symbol

The moon is seen as a symbol of fertility in many cultures (ref). If the moon as an object is directly linked to fertility and the ability to give life, its association with the menstrual cycle becomes clear again.