Sonographic assessment of amniotic fluid volume between 11 and 24 weeks of gestation: construction of reference intervals related to gestational age.

OBJECTIVE: At present, most of the methods for sonographic assessment of amniotic fluid volume are unreliable in the second trimester of pregnancy, or else they do not present nomograms related to gestational age. DESIGN: The aim of this prospective cross-sectional study was to construct normal reference ranges of four ultrasound parameters for the evaluation of amniotic fluid volume which could be applied in the second trimester. For these parameters we calculated normal curve limits suitable for use in clinical practice. SUBJECTS: From a population of normal pregnant women between the 12th and the 24th weeks of gestation undergoing a routine ultrasound examination during 1997 at our institute, 273 were found to be suitable for the study, after the exclusion of all cases which presented any feto-maternal pathology or complications up to the 24th week. METHODS: The largest 'amniotic pocket' in a vertical direction, free of small fetal parts and umbilical cord, was measured: the maximum vertical and transverse diameters were measured on the same scan; the mean diameter and the product of the two diameters were calculated. The 'mean amniotic fluid diameter', the 'two-diameter pocket', the 'largest vertical pocket' and the 'largest transverse pocket' were the four sonographic parameters considered. RESULTS: The four parameters correlated well with gestational week and with the biparietal diameter; the normal reference intervals and normal curve were then calculated. All these parameters were found to have good intra- and interoperative reproducibility. CONCLUSIONS: We conclude that the use of an ultrasound semiquantitative method based on the measurement of a single amniotic fluid pocket and involving normal reference intervals according to gestational age could improve the early diagnosis of amniotic fluid variations during the second trimester, although this has yet to be confirmed by extensive clinical trials.