An abdominal hernia occurs when an organ or other piece of tissue protrudes through a weakening in one of the muscle walls that enclose the abdominal cavity. The sac that bulges through the weak area may contain a piece of intestine or fatty lining of the colon (omentum) if the hernia occurs in the abdominal wall or groin. If the hernia occurs through the diaphragm, the muscle that separates the chest from the abdomen, part of the stomach may be involved.
The abdominal wall is made up of layers of different muscles and tissues. Weak spots may develop in these layers to allow contents the abdominal cavity to protrude or herniate. The most common abdominal hernias are in the groin (inguinal hernias), in the diaphragm (hiatal hernias), and the belly button (umbilicus). Hernias may be present at birth (congenital), or they may develop at any time thereafter (acquired).