The concentration of lecithin relative to sphingomyelin begins to rise in the following week:

Before 34 weeks, lecithin and sphingomyelin are present in amnionic fluid in similar concentrations. At 32 to 34 weeks, the concentration of lecithin relative to sphingomyelin begins to rise. Dipalmitoylphosphatidylcholine--DPPC, that is, lecithin--along with phosphatidylinositol and especially phosphatidylglycerol is an impoant component of the surface-active layer that prevents alveolar collapse. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 29. Diseases and Injuries of the Fetus and Newborn. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.