Accuracy of sonographic fetal weight estimation: a matter of presentation

To assess the accuracy of sonographic weight estimation for fetuses in breech presentation, and to determine whether certain sonographic models perform better than others in cases of breech presentation.

This was a retrospective cohort study of all sonographic weight estimations of fetuses in breech presentation performed within 3 days prior to delivery (n = 165). The accuracy of weight estimation was evaluated using eight sonographic models and was compared with a control group of fetuses in vertex presentation matched to the study group by birth weight and fetal gender (n = 165).

After exclusion of a model based on femur diaphysis length alone, which was found to be highly inaccurate, the systematic error for fetuses in breech presentation was smaller than that observed for fetuses in vertex presentation (mean of all models - 0.14% vs. 2.0%, P = 0.01). The random error was higher in cases of breech presentation (9.4-13.2% vs. 7.5-8.9%, P < 0.05), and was lower for models that are based on three to four biometric indices (9.4-9.5%) compared with models that incorporate one to two biometric indices (10.8-13.6%, P < 0.05). Overall, the higher random error outweighed the decrease in the systematic error, as reflected by the lower fraction of weight estimations within 10% of birth weight among fetuses in breech presentation. Models based on three to four biometric indices were more accurate in detecting the weight thresholds beyond which a trial of vaginal delivery is usually not recommended.

Sonographic weight estimation appears to be less accurate for fetuses in breech presentation, principally because of a larger random error. The use of sonographic models that are based on three to four biometric indices, training aimed at improving measurement in breech fetuses and the development of sonographic models derived from fetuses in breech presentation, may improve the accuracy of weight estimation in these cases.