Although<unk> , aplasia and hypoplasia are lethal congenital disorders all resulting from underdevelopment of lungs,<unk>  and aplasia differ from<unk>  hypoplasia in their underlying cause. Unlike<unk>  hypoplasia which in most cases result from the incomplete development of lung during prenatal development,<unk>  and aplasia result from a complete developmental arrest of the primitive lung during embryonic life. The difference between<unk>  and aplasia is that<unk>  has complete absence of lung tissue, airways, and lung vessels while<unk>  aplasia has complete absence of lung tissue and lung vessels, but have some incompletely developed short airways. Whether the disorder is bilateral or unilateral depends on the stage in which the arrest occurs during the embryonic stage of lung development. The earlier the occurrence of developmental arrest, the more severe the defect and the more likely that the agenesis will be bilateral. Bilateral<unk>  is highly rare and it is caused by the complete failure in the development of respiratory primordium, the primary structure developed in the earliest stage of embryonic development that gives rise to the entire respiratory tract. In this case, the absence of lung buds or pleural cavities is observed, thus making the case of bilateral<unk>  highly lethal. On the other hand, unilateral<unk>  is caused by the imbalance in the development of lung buds. Either one side of the lung fails completely, leading to<unk>  aplasia, or one side of the lung is underdeveloped, leading to dysplasia or hypoplasia. These defects occur during the early stage of lung development, although not as early as bilateral<unk> , when respiratory primordium bifurcates into right and left primitive lung buds at the end of the fourth week of gestation. Although the exact cause of the disorder remains obscure, theories have been advanced throughout history to explain the pathogenesis of lung agenesis. Based on an in vitro experiment done in rats, researchers observed lung aplasia in animals that were fed with a diet deficient in vitamin A. From the result of this experiment, a theory was raised that vitamin A deficiency during pregnancy may cause<unk> . Some authors have suggested vascular cause of<unk>  similar to the causes for intestinal atresia and others suggested Iatrogenic and viral factors as potential causes of<unk> . Genetic cause for<unk>  has also been raised. It has been said that the gene responsible for the cause of<unk>  may have variable expressivity and penetrance.
Pulmonary agenesis
Pulmonary atresia
Tracheal agenesis
Pulmonary hypoplasia
Situs ambiguus
Renal agenesis
Congenital diaphragmatic hernia
Double aortic arch