A congenital (present from birth) diaphragmatic hernia is caused by a defect in which the diaphragm, the large dome-shaped muscle that separates the chest cavity from the abdomen, fails to completely develop. The condition requires immediate surgery to repair the damage caused by the defect. The incomplete diaphragm has a large hole through which the abdominal organs can pass (herniate) into the chest cavity. Most commonly the hole occurs on the left side (almost 90% of cases). This compresses the fetal lung nearest the hernia, preventing full development, and thus interferes with breathing after the infant is born. The incidence of congenital diaphragmatic hernia is approximately 1 in 2,000 births, and boys are more commonly affected.