A new formula for estimating fetal weight: The impression of biparietal diameter, abdominal circumference, mid-thigh soft tissue thickness and femoral length on birth weight

Background: Abdominal circumference (AC), biparietal diameters (BPD) and femoral length (FL) are now the main parameters used to obtain estimated fetal weight (EFW). Although the role of soft tissue parameters in determining fetal weight was proved but clinical attention to mid-thigh soft tissue thickness (STT) is limited. Objective: To find the impression of STT on birth weight (BW) and represent a new predictive formula. Materials and Methods: One hundred and fourteen normal singleton term (36-42w) pregnancies with delivery within 72 hours were randomly selected to participate in this prospective cohort study. Variables measured by ultrasonography before birth included: AC, BPD, FL and STT. The actual neonatal BW was also measured after birth. Linear regression model was used and R square and p-value were reported. Results: The mean (SD) of BW was 3406 (405) gr. R square was best fit for the model that STT was added to AC, BPD, FL (r2: 0.77). R square for the model using BPD, AC, FL and model using BPD, STT, FL was the same (r2: 0.7). Best fit formula was Log (BW)= 2.461+0.003BPD+0.001AC+0.007STT+0.005FL. AC (R: 0.67, p < 0.001), STT (R: 0.50, p < 0.001), BPD (R: 0.59, p < 0.001), FL (R: 0.66, p < 0.001) were significantly correlated with birth weight. AC had also significant correlation with STT (p=0.001) Conclusion: This study showed adding STT to other variables in predictive models of fetal weight would provide a nice estimation (r2=0.77) and in cases that measuring AC is suboptimal STT may be a good replacement.