Surfactant is a complex mixture of compounds, with a significant amount of phospholipid. Lecithin is one of the primary phospholipids present, and its concentration in the amniotic fluid increases as the fetus matures. Sphingomyelin is another phospholipid present but its concentration tends to remain constant during the third trimester. It can therefore be used to identify the relative amount of lecithin present. The ratio of lecithin-to-sphingomyelin (the L/S ratio) of the amniotic fluid is used to assess the fetus for lung maturity which is important in gauging the risk for the respiratory distress syndrome in the infant after delivery.

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