Maturity of the foetal lungs

Surfactant is secreted by type II pneumocytes and prevents recurrent collapse of alveoli with each expiration. A major component of surfactant is the phospholipid lecithin.

The amniotic fluid concentration of lecithin increases as the lungs mature. The concentration also depends on the total volume of the liquor hence an internal control is needed. The control substance is another phospholipid, sphingomyelin, which is produced at a roughly continuous rate through gestation.

When the L:S ratio is greater than 2 then the production of surfactant is likely to be sufficient to prevent the development of respiratory distress.