Fetal maturity: biochemical analyses of amniotic fluid.

One hundred and three samples of amniotic fluid from 21 normal and 64 medically complicated pregnancies were used for evaluating fetal maturity by measuring one or more of the following: the lecithin-sphingomyelin ratio (L/S), bilirubin concentration, creatinine concentration, and percentage of fetal fat cells. The rapid foam test for surfactant was evaluated in 20 of the samples and found to be unreliable in predicting the risk of respiratory distress syndrome. The test for fat cells gave a large percentage of false negative results, and the creatinine concentration failed to show a good correlation with gestational age. Both the bilirubin concentration and L/S ratio showed good correlation with gestational age. However, since the major cause of death in a premature infant is the respiratory distress syndrome, the maturity of the fetal lung, which is best estimated by the L/S ratio, has to be the prime concern.