Each month, the uterus prepares for a possible pregnancy. The lining of the uterus thickens into a cushion of blood vessels, glands and liquid. When a female becomes pregnant, this lining will nourish the fertilised egg for the term of the pregnancy. If the egg is not fertilised, then the lining (mostly blood and some fluid) passes out through the vagina. This bleeding is called a period, or menstruation.