Pulmonary hemorrhage is defined on pathologic examination as the presence of erythrocytes in the alveoli and/or lung interstitium, with those infants surviving longer than 24 hours showing a predominance of interstitial hemorrhage. Confluent hemorrhage involving at least two lobes of the lung is termed massive pulmonary hemorrhage. There is less agreement regarding the clinical definition. Commonly, pulmonary hemorrhage is defined as the presence of hemorrhagic fluid in the trachea accompanied by respiratory decompensation requiring increased respiratory support or intubation within 60 minutes of the appearance of fluid.

The precise mechanisms underlying pulmonary hemorrhage remain uncertain. Pulmonary hemorrhage likely results from heterogeneous conditions converging in a common final physiologic pathway.

Clinically apparent pulmonary hemorrhage occurs at a rate of 1 to 12 per 1,000 live births. Accurate incidence rates are difficult to ascertain as the clinical definition is not uniform and definitive diagnosis requires pathologic examination (which may be unavailable because the event was not fatal or permission for pathologic examination was not obtained). In high-risk groups such as premature and growth-restricted infants, the incidence increases to as many as 50 per 1,000 live births. In autopsy studies, pulmonary hemorrhage is much more prevalent. Some studies report hemorrhage in up to 68% of autopsied neonates, with severe pulmonary hemorrhage occurring in 19% of infants dying in the first week of life. In most cases, death occurred 2 to 4 days after birth.