Amniotic Fluid Microviscosity Determined by Fluorescence Polarization: Methodology and Relation to Gestational Age

The fluorescence polarization of 116 amniotic fluid specimens obtained from 22 isoimmunized pregnant women was determined. The degree of fluorescence polarization of amniotic fluid provides an index of microvisocity in lipid aggregates that is dependent on the lecithin-to-sphingomyelin ratio and the degree of saturation of fatty acid side chains. We confirmed the reproducibility of the measurement of amniotic fluid microviscosity (coefficient of variation, 2.0%). The measurements are not effected by bilirubin concentration or amniotic fluid dilution. The pattern of change of amniotic fluid microviscosity during gestation parallels the expected development of the surfactant system. Amniotic fluid microviscosity is high during early gestation and abruptly and sequentially decreases between the 28th and 36th week of gestation. Since the measurements are an accurate reflection of the biochemical properties of amniotic fluid lipids and parallel the development of the surfactant system, we conclude that amniotic fluid microviscosity may well serve as an indicator of the process of fetal lung maturation.