The monthly cycle governing a female's fertility, usually from 21-35 days, and often following the lunar tide. During the bleeding phase of the cycle (3-5 d.) the lining of the uterus is sloughed off. Approx. 2 wks later, ovulation occurs and the period of a female's greatest fertility (3-5 d). Usually beginning at age 13 and ending around age 50, but this varies greatly.