Accuracy of immediate antepartum ultrasound estimated fetal weight and its impact on mode of delivery and outcome - a cohort analysis.

BACKGROUND: The aim of the study was to investigate the accuracy of ultrasound-derived estimated fetal weight (EFW) and to determine its impact on management and outcome of delivery.

METHODS: In this single-center cohort analysis, women with a singleton term pregnancy in the beginning stages of labor were included. Women with immediately antepartum EFW (N = 492) were compared to women without ultrasound (N = 515).

RESULTS: EFW was correct (deviation from birth weight ≤ 10%) in 72.2% (355/492) of patients with fetal biometry; 19.7% (97/492) were underestimated, and 8.1% (40/492) were overestimated. Newborns with a lower birth weight were more frequently overestimated, and newborns with higher birth weight were more frequently underestimated. The mean difference between EFW and real birth weight was - 114.5 g (standard deviation ±313 g, 95% confidence interval 87.1-142.0). The rate of non-reassuring fetal heart tracing (9.8% vs. 1.9%, P < 0.001) and of caesarean delivery (9.1% vs. 5.0%, P = 0.013) was higher in women with EFW. Overestimation was associated with an increased risk for delivery by caesarean section (odds ratio 2.80; 95% confidence interval 1.2-6.5, P = 0.017). After adjustment, EFW remained associated with increased non-reassuring fetal heart tracing (odds ratio 4.73; 95% confidence interval 2.3-9.6) and caesarean delivery (odds ratio 1.86; 95% confidence interval 1.1-3.1). The incidence of perineal tears of grade 3/4, shoulder dystocia, postnatal depression and neonatal acidosis did not differ between groups.

CONCLUSIONS: Antepartum ultrasound-derived EFW does not improve maternal and fetal outcome and is therefore not recommended.