A diaphragmatic hernia is caused by the improper fusion of structures during fetal development. The abdominal organs such as the stomach, small intestine, spleen, part of the liver, and the kidney appear in the chest cavity. The lung tissue on the affected side is thus not allowed to completely develop. Respiratory distress usually develops shortly after the baby is born, because of ineffective movement of the diaphragm and crowding of the lung tissue, which causes collapse. The reason why this occurs is not known. Congenital diaphragmatic hernia is seen in 1/2200 to 1/5000 live births with the vast majority (80 to 90%) occurring on the left side. There is a 2% recurrence rate in first degree relatives of a patient with the disease.
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