In a female reproductive ovaries, the ova begin to mature with the onset of puberty. One ovum matures and released by one of the ovaries once in about 28 to 30 days. During this period, the wall of the uterus becomes thick so as to receive the egg, in case it is fertilized and begins to develop. If fertilization does not occur, the released egg and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in female which is called menstruation. Menstruation occurs once in about 28 to 30 days.