Fetal Lung Maturity Testing
If a neonate is delivered before the lungs have had time to fully mature, neonatal respiratory distress syndrome (RDS, also called hyaline membrane disease) may result. A major element in the development of RDS is the inability of immature lungs to produce pulmonary surfactant, the lipid/protein material that reduces surface tension in the alveolar water layer and promotes uniform inflation of the lungs.
Testing for lung maturity is important in management of premature labor, hemolytic disease of the newborn, premature rupture of membranes, and any other clinical situations in which early delivery of the fetus would be considered. The results of these tests are used to determine whether to attempt to suppress labor or to induce delivery. If delivery cannot be delayed when the lungs are immature, dexamethasone is often given to stimulate maturation of the surfactant production system.
Pulmonary surfactant lipids
Pulmonary surfactant is rich in disaturated phophatidyl choline (lecithin). Lecithin synthesis in the lung increases dramatically as the lung matures and begins producing surfactant. Because the amount of amniotic fluid and the concentration of material in it may vary between pregnancies, lecithin levels are generally expressed as a ratio against sphingomyelin, a non-pulmonary lipid whose concentration is relatively constant in amniotic fluid. Thus the lecithin level is generally expressed as a lecithin-to-sphingomyelin ratio or L/S ratio. L/S ratios of greater than 2 are rarely associated with RDS, except in complicated pregnancies (diabetes mellitus, premature rupture of membranes, etc.). L/S ratios of less than 2 indicate increased risk of RDS, but the majority of these neonates still do not develop RDS.
Lecithin and sphingomyelin time course in amniotic fluid: