I) The uterus prepares itself every month in the hope of recieving a fertilised egg. The blood lining thickens which is richly supplied with blood vessels necessary for the nourishment of the embryo.
ii) In case the egg is not fertilised, the blood lining breaks as it is not needed anymore and exits the body in the form of blood and mucous via the vagina. This is known as menstruation. It occurs 14 days after ovulation.