Amniotic fluid volume estimation in the postdate pregnancy: A comparison of techniques

Objective: To determine the amniotic fluid (AF) volume estimation technique with the greatest diagnostic value with respect to perinatal outcome in the postdate pregnancy. Methods: One hundred ninety-eight women who were at least 40 weeks' gestation based on ultrasound confirmation or establishment of dates were evaluated twice weekly with nonstress tests (NSTs) and AF volume estimation. Various indices of AP volume were measured and correlated with perinatal outcome. Receiver operating characteristic curves were used to determine optimal cutoff values. Results: There was a significantly larger maximal vertical pocket in those pregnancies with a normal perinatal outcome than in those with abnormal outcome (4.2 ± 1.4 versus 3.2 ± 2.0 cm; P = .02). A statistically significant difference was also seen with the AF index (10.0 ± 4.5 versus 7.5 ± 5.0 cm; P = .01). Analysis revealed that the optimal cutoff for identifying clinically significant oligohydramnios was a largest vertical pocket less than 2.7 cm, with an adjusted odds ratio for abnormal perinatal outcome of 7.11 (95% confidence interval [CI] 2.79-18.16). Its sensitivity of 50.0% and specificity of 87.9% were superior to the diagnostic value of the AF index of 5 cm, which had a sensitivity of 29.2% and specificity of 89.1%. Conclusion: A largest vertical pocket threshold of 2.7 cm had the greatest diagnostic value for identifying the postdate pregnancy at risk for abnormal perinatal outcome, surpassing the AF index and the 2-cm largest vertical pocket rule.