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Assessment of foetal lung maturity by gas chromatographic analysis of the fatty acids in the amniotic fluid. Statistical reliability of the technique. Gas-chromatographic analysis of the fatty acids (Palmitis acid/Steric acid ratio: P/S ratio) was carried out in 432 samples of amniotic fluid obtained between the 28th and 42th weeks of gestational age. The present results from a series of normal (257) and various pathological (175) cases support the conclusions of other investigations that an amniotic fluid P/S ratio equal to or greater than 5 is a reliable indicator of attained fetal lung maturity. When correlated with gestational age the P/S ratios show regression lines that define the range of lung maturity in normal pregnancies between 32 and 33 weeks and in pathological cases around and over 36 weeks. Statistical studies stress that centrifugation in standard sample conditions before its gas chromatographic analysis is an essential step in order to avoid the serious consequences of an erroneous prediction. Finally Lecithin/Sphingomyelin (L/S) and P/S ratios were assessed in the same 100 samples of amniotic fluid: statistical analysis shows a very good correlation between the two methods and their reliability in the prediction of fetal lung maturity.