The lecithin sphingomyelin (L/S) ratio is measured for assessing maturity of the following organ:

The lecithin/sphingomyelin (L/S) ratio for assessment of fetal pulmonary maturity. Before 34 weeks, lecithin and sphingomyelin are present in amniotic fluid in similar concentrations. At 32 to 34 weeks, the concentration of lecithin relative to sphingomyelin begins to rise. In the absence of complications, the ratio of these 2 components reaches 2.0 at approximately 35 weeks. There is increased risk of respiratory distress when this ratio is below 2. Because lecithin and sphingomyelin are found in blood and meconium, contamination with these substances may lower a mature L/S ratio. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 29. Diseases and Injuries of the Fetus and Newborn. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.