Meckel's diverticulum is a small pouch in the wall of the lower small intestine. The pouch is a congenital (present at birth) abnormality that affects approximately 2% of the population. The pouch, or diverticulum, seems to be leftover tissue from the development of the digestive system. This tissue normally is reabsorbed before birth. When it contains gastric (stomach) tissue, it is more likely to cause bleeding.