Determination of fetal lung maturity from amniotic fluid microviscosity in high-risk pregnancy.

During a 3-year period, 377 samples of amniotic fluid, free of blood and meconium and obtained from a total of 471 patients, were examined by the fluorescence polarization technique to estimate fetal lung maturity. Fluorescence polarization values of 287 samples from normal and abnormal pregnancies were correlated with gestational age. A total of 272 pregnancies comprising cases complicated by toxemia, placental insufficiency, placenta previa, premature contractions, and diabetes mellitus, and including a control group of 37, were studied to discern the effect of such pathologic conditions on the usual decline of fluorescence polarization (FP) values with increasing gestational age. No complicated pregnancy had values significantly different from those of the control group, except for pregnancies complicated by diabetes mellitus. The threshold of fetal lung maturity as related to respiratory distress syndrome (RDS) was determined in correlation with the lecithin: sphingomyelin (L:S) ratio. FP values of 0.311 +/- .005 corresponded to an L:S ratio of 2.0. An FP value of 0.316 +/- .005 is recommended as a practical threshold related to the clinical outcome of the newborn as regards RDS. The technique was found reliable, simple, and accurate and has been of great value in the antenatal diagnosis of fetal lung maturity.