[Optical density and surface tension in amniotic fluid in the late pregnancy (author's transl)].

113 amniotic fluid samples from pregnant women between the 28th and the 41st week of gestation were examined for optical density and surface tension. Surface tension measurement was done by means of the Wilhelmy-balance which shows a very good correlation with the L/S-ratio. Optical density was measured at 405, 580, 650, 750 and 900 nm wave length. For the comparison with the surface tension optical 650 nm was used, since at this wave length only minimal absorbance of known biologic pigments occurs. Pulmonary maturity was diagnosed when the surface of the hysteresis area (dynamic surface tension measurement) was more than 20 cm2 or when optical density at 650 nm was false positive in 3,7% and false negative in 40,7%. The same percentage was found in the predication of fetal lung maturity by this method after fetal outcome. The measurement of the optical density is a rapid and simple method, but because of the high number of false negative results it is advised to use one of the more sophisticated methods for the prediction of fetal lung maturity in cases of a negative optical density reading at 650 nm.