Why do we menstruate? 

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Every month, the uterus (where a baby would grow if a person becomes pregnant) prepares for a possible pregnancy by building up a thick lining of tissue. This lining is to support a fertilized egg. If there is no fertilized egg, the body doesn't need this extra tissue, so it gets rid of it. This is what causes menstruation, or a period. The body sheds the extra tissue and blood through the vagina. This cycle usually repeats about every 21 to 35 days.