Congenital diaphragmatic hernia is an abnormality that occurs before birth as a fetus is forming in the mother's uterus. The specific defect is an opening or whole in the diaphragm (the muscle that separates the chest cavity from the abdominal cavity).
With this defect, some of the organs that normally are found in the abdomen move up into the chest cavity through this abnormal opening. The herniation of these abdominal organs into the chest occupies that space and prevents the lungs from growing to normal size.
This occurs in about 1 out of every 2,500 live births and the cause is not known. Survival rate is about 50%, but this varies greatly with each individual baby- depending on their amount of lung development at birth, timing and type of treatment given, presence of any other complications, and many other things.