The lecithin–sphingomyelin ratio (L-S or L/S ratio) is a test of fetal amniotic fluid to assess for fetal lung immaturity. Lungs require surfactant to lower the surface pressure of the alveoli in the lungs. This is especially important for premature babies trying to expand their lungs after birth. Surfactant is a mixture of lipids, proteins, and glycoproteins, lecithin and sphingomyelin being two of them. Phosphatidylcholine, a type of phospholipid in lecithin pictured.