Waist Circumference as a Predictor of Failure of Sonographic Estimation of Fetal Birth Weight.

Objective: Several factors may interfere with the success of fetal birth weight (BW) prediction. In this study we tried to determine the most probable factors that may lead to unsuccessful BW estimation.

Methods: 200 consecutive pregnancies between 34 and 41 weeks of gestation were enrolled for the study. All subjects underwent sonographic fetal BW estimation before membrane rupture or engagement of presented part. Sonography was performed by the same sonographer blinded to the study design. Failure of estimation was determined when deviation was found to be >15%.

Results: Both amniotic fluid index (AUC = 0.768, p < 0.001) and maternal waist circumference (AUC = 0.698, p = 0.004) were significant predictors for failure of estimation. Optimal cut-off values were 80 mm for amniotic fluid index (77% sensitivity, 65% specificity) and 105 cm for maternal waist circumference (70% sensitivity, 61% specificity). The number of pregnancies with anteriorly located placenta was significantly higher in the group with failed estimation (12/20 vs. 39/180, p = 0.001).

Conclusion: Amniotic fluid volume, body mass index, maternal waist circumference and placental location may all cause failure of fetal weight estimation and may need to be adjusted. Moreover, our results indicate that waist circumference may be a more reliable predictor of failure of fetal weight estimation compared to body mass index.