Estimation of fetal weight by ultrasonography after preterm premature rupture of membranes: comparison of different formulas.

Objective: To compare different ultrasonographic fetal weight estimation formulas in predicting the fetal birth weight of preterm premature rupture of membrane (PPROM) fetuses. Methods: Based on the ultrasonographic measurements, the estimated fetal weight (EFW) was calculated according to the published formulas. The comparisons used estimated birth weight (EBW) and observed birth weight (OBW) to calculate the mean absolute percentage error [(EBW - OBW)/OBW x 100], mean percentage error [(EBW - OBW)/OBW x 100)] and their 95% confidence intervals. Results: There were 234 PPROM patients in the study period. The mean gestational age at which PPROM occured was 31.2 +/- 3.7 weeks and the mean gestational age of delivery was 32.4 +/- 3.2 weeks. The mean birth weight was 1892 +/- 610 g. The median absolute percentage error for 33 formulas was 11.7%. 87.9% and 21.2% of the formulas yielded inaccurate results when the cut-off values for median absolute percentage error were 10% and 15%, respectively. The Vintzileos' formula was the only method which had less than or equal to 10% absolute percentage error in all age and weight groups. Conclusions: For PPROM patients, most of the formulas designed for sonographic fetal weight estimation had acceptable performance. The Vintzileos' method was the only formula having less than 10% absolute percentage error in all gestational age and weight groups; therefore, it may be the preferred method in this cohort. Amniotic fluid index (AFI) before delivery had no impact on the performance of the formulas in terms of mean percentage errors.