Fetal mid-thigh soft-tissue thickness: a novel method for fetal weight estimation

To derive a novel formula for fetal weight estimation utilizing the linear measurement of mid-thigh soft-tissue thickness (STT).

300 women, with singleton uncomplicated pregnancy, were included in a prospective cross-sectional study. The study included four consecutive phases: phase (1) validated the original Scioscia's formula, phase (2) derived a novel modified formula for fetal weight estimation, phase (3) validated the novel modified formula, and phase (4) evaluated the agreement between the modified and original formulae.

A statistically significant correlation was found between actual fetal weight (AFW) and various sonographic biometric measurements including mid-thigh STT (r (2) = 0.656, p < 0.001), femur length (FL) (r (2) = 0.573, p < 0.001), bi-parietal diameter (BPD) (r (2) = 0.250, p < 0.001), abdominal circumference (AC) (r (2) = 0.310, p < 0.001), and estimated fetal weight (EFW) using the original Scioscia's formula (r (2) = 0.644, p < 0.001). The modified formula showed a better signed % difference (median = -0.41 %, IQR -1.88 to 2.03) than the original formula (median = -0.51 %, IQR -2.33 to 2.00). It was noted that, using the original formula, 88.7 % of the sample had absolute error below 5 and 98.3 % of the sample had absolute error below 10 %. On the other hand, using the modified formula, 87.3 % of the sample had absolute error below 5 %, while 97.3 % had absolute error below 10 %. The agreement between the two formulae was moderate as 134 patients out of 150 had similar ranking (κ = 0.57).

Fetal mid-thigh SST is a simple, useful, and easily applicable parameter for fetal weight estimation.