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Tumors, scar tissue (adhesions), or twisting or narrowing of the intestines can cause a bowel obstruction. These are called mechanical obstructions.
In the small intestine, scar tissue is most often the cause. Other causes include hernias and Crohn's disease, which can twist or narrow the intestine, and tumors, which can block the intestine. A blockage also can happen if one part of the intestine folds like a telescope into another part, which is called intussusception.
In the large intestine, cancer is most often the cause. Other causes are severe constipation from a hard mass of stool, and narrowing of the intestine caused by diverticulitis or inflammatory bowel disease.