Congenital diaphragmatic hernia occurs once in 2200 births. It is caused by a defect in the hemidiaphragm, usually the left. It is unusual amongst congenital anomolies in that it is most often isolated - there are rarely other problems with the baby.

Note that very rarely, small congenital diaphragmatic hernia may present late - up to three years of age - with collapse and apparent sepsis.