Postmortem lung weight/body weight standards for term and preterm infants

Assessment of lung development is a crucial component of the perinatal autopsy, especially in preterm infants. In current pathology practice, the diagnosis of lung hypoplasia in very premature infants (<28 weeks of gestation) is based on a lung weight/body weight ratio (LW/BW) of 1.5% or less. While useful to confirm the presence of marked pulmonary hypoplasia, the 1.5% threshold may be too stringent to detect more subtle degrees of pulmonary hypoplasia, as seen in association with rupture of membranes. The aim of this study was to establish reference values for postmortem LW/BW in preterm and term infants. To this end, we performed a retrospective analysis of fetuses and newborns (16-41 weeks of gestation) without known risk factors for pulmonary hypoplasia. Mean LW/BW ranged between 2.98-3.15% between 16-27 weeks, and decreased to 2.55% by 28-36 weeks and 1.79% at term. The 10th percentile for LW/BW was significantly higher in preterm infants (2.24% between 20-23 weeks, and 2.59% between 24-27 weeks) than in term infants (1.24% at >or=37 weeks, P < 0.01). We then correlated LW/BW of infants with risk factors for pulmonary hypoplasia with the newly established reference values. As expected, mean LW/BW of infants with congenital diaphragmatic hernia or severe renal anomalies was <1.5% (0.98% and 1.40%, respectively). Mean LW/BW of infants with prolonged (> 1 week) rupture of membranes (PROM) was 2.08%. Three of 12 (25%) preterm infants with PROM had pulmonary hypoplasia according to the traditional criteria (LW/BW <1.5%). However, an additional 4/12 (33%) infants with PROM had a LW/BW between 1.5% and the 10th percentile for age (2.2%), indicative of more subtle but potentially relevant pulmonary underdevelopment. In conclusion, we determined LW/BW reference values for preterm and term infants. Correlation of LW/BW with age-matched reference values at postmortem examination represents an invaluable additional tool to evaluate lung growth, particularly in preterm infants.