In every month as one of the ovaries releases ovum and tfue uterine wall thickens and prepares itself for receiving the developing zygote. When the egg is not fertilised, the level of progesterone starts decreasing, due to which nourishment to the endometrium is stopped. As a result, the lining of uterine wall slowly breaks and the discharge of unfertilised egg along with ruptured endometrium and mucus causes menstruation in a human female.