Also where the English words "month" and "menses" come from. Fun fact: "menstrual" simply means monthly, and is equivalent to "journal" or "annual". Its association with female reproductive cycles is euphemistic!
In Portuguese, the logic is the same once "month" is "mês".
Another funny fact: Venus, often seen as a symbol to feminine nature, takes 38 weeks to turn a year (about 9 and half months), and that's also the time of a normal pregnancy.