Ultrasonographic assessment of amniotic fluid does not reflect actual amniotic fluid volume

OBJECTIVE: Our objective was to compare the ability of two methods of amniotic fluid assessment (two-diameter amniotic fluid pocket versus the amniotic fluid index) to predict oligohydramnios (actual amniotic fluid volume <500 ml) or polyhydramnios (actual amniotic fluid volume >1500 ml). STUDY DESIGN: The amniotic fluid index and the two-diameter amniotic fluid Docket were assessed before amniocentesis and determination of amniotic fluid volume with the dye (aminohippurate sodium)-dilution technique. To assess the detection of either oligohydramnios or polyhydramnios, the areas under the receiver-operator characteristic curves (±SE) were estimated by the point- to-point trapezoidal method of integration. Prediction limits were calculated by regression analysis of amniotic fluid index or two-diameter amniotic fluid pocket versus actual amniotic fluid volume and determination of 95th percentile ranges for amniotic fluid volume. RESULTS: We studied 144 patients with a mean (±SD) gestational age of 31.7 ± 5.5 weeks; mean (±SD) amniotic fluid index and two-diameter amniotic fluid pocket were 12.6 ± 6.1 cm and 21.2 ± 18.4 cm², respectively. Mean (±SD) actual amniotic fluid volume was 722 ± 735 ml (range 101 to 4318 ml). The areas under the four receiver- operator characteristic curves were not significantly different from the nondiagnostic line (p < 0.05). Regression slopes (r values) for amniotic fluid index and two-diameter amniotic fluid pocket versus actual amniotic fluid volume were 0.34 and 0.23, respectively. Calculation of the prediction limit for 95% confidence that oligohydramnios is absent requires that the amniotic fluid index be 30 cm and the two-dimension amniotic fluid Docket be 90 cm², both thresholds of which are currently considered to represent clinical polyhydramnios. CONCLUSIONS: Both amniotic fluid index and two- dimension amniotic fluid pocket appear to be inaccurate predictors of actual oligohydramnios or polyhydramnios when compared with dye-dilution calculations of actual amniotic fluid volume.