Comparison of sonographic fetal weight estimation formulas in patients with preterm premature rupture of membranes

Objectives Numerous formulas for estimated fetal weight (EFW) calculations have been proposed over the years but have not been validated in patients with preterm premature rupture of membranes (PPROM). The study objective was to compare the accuracy of the various sonographic EFW formulas in patients with PPROM. Methods This was a retrospective cohort study of women with PPROM and singleton gestations who were admitted between 220–330 weeks to a single, university-affiliated tertiary referral centre between 2003–2017. All women had a sonogram for EFW within 14 days of delivery by standard biometry (biparietal diameter, head circumference, abdominal circumference, and femur length). We compared the accuracy of 21 previously published EFW formulas in the setting of PPROM by comparing the correlation with actual birth weight and calculating the systematic error (SE), random error (RE), proportion of estimates (POE) within 10% of birth weight and Euclidean distance. Results Overall, 565 women were included in the study. Most formulas had strong correlation with actual birth weight (19/21 formulas with r >0.9). The mean SE was -4.30% and mean RE 14.48%. The formula that performed the best, by the highest POE and lowest Euclidean distance was that of Ott (1986), utilizing abdominal and head circumferences, and femur length, yet all of the first 10 ranking formulas (including the mostly used Hadlock IV) performed quite similarly with minimal differences between them. Conclusions Most formulas accurately calculation EFW in PPROM patients, but the Ott (1986) formula ranked highest, thus it may be the preferred formula to use in this population.