Congenital diaphragmatic hernia (CDH) can be diagnosed in the prenatal period either as part of other anomalies or as an isolated birth defect. The clinical impact of this surgically correctable anatomical defect lies in its impairment of lung development. Currently, up to 30% of babies with isolated CDH die from the consequences of lung hypoplasia and/or pulmonary hypertension. Antenatal prediction of outcome essentially relies on the measurement of lung development by the so-called lung area to head circumference ratio (LHR). By expressing observed LHR as a proportion of what is normally expected (O/E LHR) at a certain time point in gestation, a prediction of outcome can be made. When O/E LHR is less than 25% of the normal, postnatal death is very likely. In these cases, an antenatal intervention that can improve lung development is currently offered. Currently, this is done by percutaneous fetal endoscopic tracheal occlusion (FETO) with a balloon at 26-28 weeks, and reversal of occlusion at 34 weeks. The feasibility and safety of percutaneous FETO have been established and the procedure seems to improve outcome in severe CDH. The lung response to, and outcome after, FETO depend on pre-existing lung size respectively gestational age at birth. Prenatal decision making can therefore be stratified according to measured lung size.