Pulmonary hypoplasia (PH) is defined as defective or incomplete develo
pment of lungs that are immature for gestational age. A prospective st
udy was done to establish practical criteria for the diagnosis of pulm
onary hypoplasia and to determine the relative frequency of pulmonary
hypoplasia and its associated congenital malformations. Postmortem, fo
rmalin-inflated, routinely processed lungs were examined for fresh lun
g weight, fixed lung volume, radial alveolar count, and tissue maturit
y. Of these, lung volume was found to be the least useful. However, no
single parameter was adequate for diagnosis in every case. Using the
remaining three in conjunction, reliable diagnostic criteria were esta
blished. In this series of 113 cases, 22 % had pulmonary hypoplasia as
the primary cause of death, of which one-third had no associated cong
enital malformations to account for the development of pulmonary hypop
lasia. It was found that premature rupture of membranes (PROM) for as
short as 5 days could lead to fatal PH. It is concluded that PH is a c
ommon problem in neonatal autopsies and can be diagnosed in the averag
e laboratory. Recognizing PH is important because once it has develope
d, intrauterine intervention in cases of oligohydramnios and postnatal
treatment with surfactant inhalation cannot influence the outcome. It
remains to be seen whether early fluid replacement in PROM will preve
nt development of PH.