The periodic series of changes in the female reproductive system associated with the preparation of the uterus for pregnancy; the cycle is repeated roughly every twenty-eight days. During the menstrual cycle, an ovum is released from one of the ovaries (the release is called ovulation), and the uterus develops an inner lining enriched with blood to prepare it for the possible implantation of a zygote. If fertilization and implantation do not take place, the lining of the uterus is discharged during menstruation.