This describes the changes which normally take place in a more or less monthly cycle in the ovaries and the lining of the womb. During a normal menstrual cycle, the ovary will release an egg and the womb lining will thicken so that the embryo can implant if the egg is fertilised. If this happens, the cycle ends in pregnancy. If the egg is not fertilised, the thickened lining is shed through the vagina - this is what causes menstruation / periods. See also 'menstruation'.