Congenital diaphragmatic hernia (CDH) is a failure in the closure of the diaphragm during early gestation. CDH occurs in approximately 1 in 3,000 live births and can be accurately diagnosed by early second-trimester ultrasound examination. The severe forms of isolated CDH are associated with high neonatal mortality and morbidity due to extremely reduced lung volumes, liver herniation and decreased abnormal pulmonary vascularization. Despite progress in prenatal diagnosis and postnatal therapy, congenital diaphragmatic hernia still carries a significant mortality and considerable short- and long-term morbidity.

Fetal endoscopic tracheal occlusion performed between 26 and 30 weeks (standard FETO) has been shown to increase fetal pulmonary size and vascularity, and to improve neonatal survival in isolated severe CDH. Fetal pulmonary response followed FETO can be used to predict outcome and is dependent on the size of the fetal lung prior to the procedure. Recent results showed that fetuses with isolated CDH that have lung-to-head ratio (LHR) prior to standard FETO less than 0.7 (observed/expected LHR <0.15 - extremely severe forms) have limited fetal pulmonary response and poor outcomes.