Amniotic fluid lecithin to sphingomyelin ration of 3.5 and fetal pulmonary maturity.

The clinical outcome of 118 pregnancies assessed by amniotic fluid lecithin to sphingomyelin concentration (L/S) ratio within 2 days of delivery is presented. It has been observed that the critical L/S ratio for pulmonary function is 3.5 in that no infant born with an L/ S ratio greater than 3.5 has had respiratory distress syndrome (RDS). Below an L/S ratio of 3.5, the risk and severity of RDS is inversely related to the L/S ratio. An L/S ratio smaller than 3.5 has the same prognostic value in infants born between 30 and 34 weeks gestation as it has in infants born between 35 and 38 weeks gestation. Under these conditions the relation between L/S ratio and RDS is independent of gestational age.