Why is the mortality so high for babies with congenital diaphragmatic hernias?

Poor Lungs: The main reason is that the absence of diaphragm (i.e. Diaphragmatic hernia) allows intestine to enter the chest and occupy the space into which lung would normally develop. This results in underdeveloped lung(s) which are often abnormal in other ways as well. Small abnormal lungs cause myriad severe problems after birth and contribute to almost all of the observed mortality in cdh patients.

Small lungs: Babies with diaphragmatic hernia have intestines entering the chest through the defect. The lungs do not have adequate space to develope. As a result babies have pulmonary hypoplasia that may be inadequate for normal life. Mortality used to be 50%. It has increased to 80-85% in some studies.