Menstrual cycle begins when a girl reaches the age of puberty. At this age, ovaries release sex hormones in the blood which cause maturation of ova in ovaries. One mature egg or ovum is released by one of the ovaries once in about 28 to 30 days. This is termed as ovulation. Before ovulation, uterus prepares itself to receive the fertilized egg. The inner lining of the uterus becomes thick and soft with lots of blood capillaries in it.

If the ovum does not get fertilized, then the inner lining of uterus is not required and hence it breaks down and comes out through the vagina in the form of a bleeding called menstruation.

Menstruation occurs once in about 28 to 30 days and it usually lasts for 3 to 5 days.