Phospholipids and creatinine in amniotic fluid in relation to gestational age

185 samples of amniotic fluid were obtained from 174 healthy women with uncomplicated pregnancies in the 15th-43rd week of pregnancy by abdominal amniocentesis or by puncture of the amniotic sac via an amnioscope. The concentrations of creatinine (183 cases), phospholipid phosphorus and the ratio of lecithin/sphingomyelin (L/S-ratio, 130 cases) were determined. The treatment of the amniotic fluid samples (centrifugation, filtration and lipid extraction) and the methods for the determination of L/S-ratio were examined and the accuracy and limitations of the latter method were defined and discussed. The relative amounts of lecithin and sphingomyelin as well as the L/S-ratio and creatinine concentration were closely correlated to gestational age. A L/S-ratio of greater than or equal to 2.25 indicated all women to be greater than or equal to 34th week of pregnancy; 64% of them had reached the 38th week. A creatinine concentration of greater than or equal to 1.8 mg% corresponded to X greater than or equal to 35th week or pregnancy, while 75% of the women had reached at least the 38th week. With a L/S-ratio greater than or equal to 2.25 and a creatinine concentration greater or equal to 1.8 mg% of all women were greater than or equal the 36th week. 71% of the women greater or equal the 38th week fulfilled these critiera. A combination of the two parameters seems to be of a great value in discrimination between gestational ages less than the 36th week and greater than or equal to the 38th week.