Estimation of fetal weight by measurement of fetal thigh soft‐tissue thickness in the late third trimester

Objective: The accuracy of current formulae for the sonographic estimation of fetal weight (EFW) is compromised by significant intra- and interobserver variability of biometrical measurements, particularly circumferences. The aim of this study was to assess the reliability of the linear measurement of mid-thigh soft-tissue thickness (STT) and to derive a novel formula for EFW. Methods: This was a prospective study involving 388 singleton uncomplicated pregnancies. There were three consecutive phases: (1) to verify the relationship between STT and birth weight, (2) to derive a novel formula for EFW using femur length and STT only, and (3) to test the accuracy of the new equation. Only the 290 patients who delivered within 48 h of measurement were considered for the analysis. A comparison with other formulae was performed. Results: STT was significantly correlated with both abdominal circumference and birth weight (r 2 = 0.36 and 0.46, respectively; P < 0.001). Both intra- and interobserver variability were satisfactory (0.44 ± 0.27 and 0.57 ± 0.35 mm, respectively). The equation for EFW was developed using multiple stepwise regression analysis (EFW = -1687.47 + (54.1 x femur length) + (76.68 x STT)) and tested prospectively on 69 patients. The new formula yielded results (r = 0.79) that were slightly better in accuracy than two other published equations, and had an absolute mean error of < 15% in 97% of cases. Conclusions: Our findings confirm the potential of the linear measurement of mid-thigh STT as a valuable parameter for the sonographic assessment of fetal growth and EFW. Our new equation is apparently at least as reliable as the most widely used formulae for EFW. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.