Pulmonary hypoplasia, the result of abnormal antenatal lung growth, occurs as a primary anomaly or more usually the consequence of disorders resulting in reduction of amniotic fluid volume, intra-thoracic space or fetal breathing movements. Two dimensional (2D) ultrasound examination yields useful information on amniotic fluid volume and the presence of congenital abnormalities associated with pulmonary hypoplasia. Assessment of the thoracic or lung size by 2D ultrasound examination, is insufficiently specific, but a very low lung to head ratio predicts a fatal outcome in fetuses with congenital diaphragmatic hernia (CDH). Three-D ultrasound examination provides information on fetal lung volume prior to 34 weeks of gestation but can be inaccurate if there is oligohydramnios. Magnetic resonance imaging is expensive and has limited patient acceptability. Antenatal interventions attempted to prevent pulmonary hypoplasia include amnioinfusion, which can facilitate ultrasound examination, but only temporarily relieves oligohydramnios. Thoraco-amniotic shunting results in effective drainage of pleural effusions, facilitating resuscitation, but is usually performed too late in pregnancy to influence lung growth. In utero surgical repair of CDH has been attempted, but a more promising technique is obstruction of the normal egress of fetal lung fluid by placing a balloon in the trachea.