ABSTRACT
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. To find the impression of STT on birth weight [BW] and represent a new predictive formula. 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 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 [r[2]: 0.77]. R square for the model using BPD, AC, FL and model using BPD, STT, FL was the same [r[2]: 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] This study showed adding STT to other variables in predictive models of fetal weight would provide a nice estimation [r[2]=0.77] and in cases that measuring AC is suboptimal STT may be a good replacement