This phase begins on the first day of menstruation and lasts until the 13th of the menstrual cycle. The egg cell matures within the sac-like-structure called the follicle and it will take 13 days for it to reach full maturity. While the egg cell matures, the follicle discharges a hormone that activates the uterus to develop a lining of blood vessels and soft tissue called endometrium.