Why does menstruation occur ?

In human females, one mature egg forms each month from the onset of puberty.If the egg does not get fertilized (due to non-availability of sperm in the female body) then the inner linings of the uterus which are thick and soft is no longer needed and hence it breaks. So, these linings of the uterus along with the blood vessels and the dead ovum (or egg) comes out of the vagina in the form of blood called menstruation. Menstruation occurs after the interval of every 28 days and the time period between ovulation and menstruation is about 14 days.