In females, the ova (pl.of ova) begin to mature with the onset of puberty (10 to 12 years of age). one egg matures and is released by one ovary once in about 28 to 30 days. Each ovary releases the ovum alternatively. During this period, the wall of the uterus becomes thick so that it can receive the fertilised ovum. In case it is fertilized, it begins to develop which results in pregnancy. If the fertilization doesn’t occur, the ovum or the released egg and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding from the vagina. This is called menstruation. It occurs every 28 to 30 days.