Accuracy of sonographically estimated fetal weight with and without oligohydramnios -: A case-control study

OBJECTIVE: To determine the accuracy of sonographically estimated fetal weight among women with and without oligohydramnios (amniotic fluid index [AFI] less than or equal to 5.0 cm) and to ascertain the ability to detect fetal growth restriction (FGR) (estimated birth weight <10th percentile for gestational age [GA]) among patients in two groups. STUDY DESIGN: Assuming that 50% of sonographic predictions are within 10% of the birth weight in the study group, 300 parturients are necessary to show a difference of 15% among controls (alpha=.05, beta=.02). The study group consisted of parturients with a reliable GA of greater than or equal to 24, no known anomalies and known AFI of less than or equal to 5.0 cm. The control (1:1) was the next patient with the same GA but AFI between 5.1 and 23.9 cm. The paired t test was used, and the odds ratio (OX) and 95% confidence interval (CI) were calculated. P < .05 was considered significant. RESULTS: Among the study and control groups (N = 162 each), maternal demographics, mean estimate (P = .078) and actual birth weight (P=.091) were similar. Sonographic estimates within 10% of weight were not significantly different among those with (57%) and without oligohydramnios (59%; OR 0.92; 95% CI 0.59, 1.44). The frequency of FGR was higher among those with inadequate fluid (18%) than controls (9%; OX 2.13; 95% CI 1.10, 4.16). Sensitivity, positive predictive value and likelihood ratio were higher among those with oligohydramnios (76%, 78% and 16) than controls (53%, 42% and 7). CONCLUSION: The accuracy of sonographic estimates of fetal weight is not influenced by whether the parturient has oligohydramnios. Moreover, the accuracy of identifying FGR is not diminished among those with AFI less than or equal to 5.0 versus >5.0 cm.