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This some of the information that I have on Lecithin; don't know if it's what
you are looking for:

Lecithin:  Sphingomyelin ratio.  The ratio of lecithin to the sphingomyelin in
the amniotic fluid.  It is used to assess maturity of the fetal lung.  Until
about the 34th week of gestation the lungs produce less lecithin than
sphingomyelin.  As the fetal lungs begin to mature, they produce more lecithin
than sphingomyelin.  Delivery before the reversal of the ratio is associated
with an increased risk of hyaline membrane disease in the infant.  The use of
this test enables the obstetrician to determine the optimum time for elective
termination of pregnancy.

Clelia