Pulmonary hypoplasia is a decrease in weight and/or volume of the lung due to impaired growth in utero. It is identified in approximately 14% of perinatal autopsies, and may be either primary (see Chapter 150), or in the vast majority of cases, secondary to mechanical constraints on the developing lung. Common causes of secondary pulmonary hypoplasia include oligohydramnios (bilateral renal agenesis, prolonged rupture of membranes), fetal hydrops with pleural effusions, thoracic space-occupying lesions (congenital diaphragmatic hernia, cystic adenomatoid malformation, neoplasms), small thoracic size (osteochondrodysplasia), and decreased thoracic movement (fetal akinesia sequence due to congenital myopathy or encephalopathy). Of these, congenital diaphragmatic hernia is the most common malformation associated with pulmonary hypoplasia.

The size of the lungs may be symmetric or asymmetric. In the setting of congenital diaphragmatic hernia, the lung ipsilateral to the hernia, usually the left, is much smaller than the contralateral lung, although both lungs are compressed and hypoplastic to some degree. At autopsy examination, lung hypoplasia is best assessed by calculating a lung to body weight ratio and by comparing measured lung volumes to expected values based on body size, where hypoplasia is defined as a lung-to-body weight ratio less than 0.012 for 28 weeks of gestation or greater. Severely hypoplastic lungs, for example, the ipsilateral lung in diaphragmatic hernia, often have a measured volume of 10% to 30% of expected volume.