A respiratory malformation characterized by the presence of both bronchi (albeit rudimentary) and alveoli in an under-developed lobe. Both the size and the weight of the lung are reduced. The true prevalence is not well known (1.4% of all births according to Knox et al. 13), but in cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia ranges from 9 to 28%. Factors that contribute to pulmonary hypoplasia include adequate volume of the thoracic cavity, pulmonary fluid dynamics, and abnormal fetal breathing movements.