Menstrual cycle is the reproductive cycle in all primates and begins at puberty. In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. During the middle of the menstrual cycle, one ovum is released. The cycle starts with the menstrual flow, caused due to the breakdown of the endometrium of the uterus. Blood vessels in liquid state are discharged, but this occurs only when the ovum is not fertilised.
It is followed by the follicular phase where the primary follicles mature into Graafian follicles. This causes the regenration of the endometrium. These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins increases. This causes follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle and cause the rupture of the Graafian follicles to release ovum. This phase is called the ovulatory phase. The remains of the Graafian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. In the absence of fertilization, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle.