L:S ratio

L:S ra·ti·o

lecithin:sphingomyelin ratio

(les'i-thin sfing?go-mi'a-lin ra'she-o),

L:S ratio

The ratio of lecithin to 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 best time for elective termination of pregnancy. Other tests commonly used for this purpose include the amniotic lamellar body count, phosphatidylglycerol presence, and the shake test. See: amniocentesis