How Menstrual Cycle Occurs In Females

In human females, periodic reproductive cycle completes in approximately 28 days, which involves changes in structure and function of whole reproductive system. It is called the menstrual cycle and can be divided into four phases. The events of menstrual cycle involve the ovaries (ovarian cycle) and the uterus (uterine cycle) and these are regulated by pituitary gonadotropins.

Primary steps in female reproductive cycles are:

• The pituitary gland on the on set of puberty, releases follicle stimulating hormone (FSH) which stimulates the development of follicles. Only one of these follicles continues to grow with its primary oocytes while the rest break down by a degenerative process known as atresia.
• The ovary, under the stimulus of FSH, also produces estrogen hormone. This, on one hand, stimulates the endometrium (internal living of the uterus wall) and vascularizes it, and on the other hand, inhibits the secretion of FSH from pituitary gland. 
• Decrease of FSH and increase of estrogen, causes the pituitary gland to secrete luteinizing hormone (LH) which induces ovulation - the release of ovum from the follicle.
• The follicle cells, after release of the egg, are modified to form a special structure called corpus luteum. This yellowish glandular structure starts secreting an hormone called progesterone. This hormone develops the endometrium and make it receptive for the implantation of the zygote (placenta formation).
• If fertilization does not occurs, the corpus luteum starts degenerating. The progesterone secretion diminishes and its supporting affect on spongy endometrium is reduced, which suffers a breakdown. This causes the discharge of blood and cell debris known as menstruation. This stage usually last for 3-7 days. 

The cycle is thus completed and the uterus is ready to enter  into the next cycle. The human menstrual cycle generally repeats itself every 28 days although there is considerable variation in different individuals or within the same individual at different times of her age. The end or complete stop of the menstrual cycle is called menupause, after which female stops producing ova.