Estimation of Fetal Weight by Measuring Abdominal Circumference, Abdominal Subcutaneous Soft Tissue Thickness, Femur Length and Mid-thigh Soft Tissue Thickness

Overview

Traditional methods for estimating fetal weight (EFW) often miss the mark because they focus on bone and circumference while ignoring fetal fat. This study proposes a more inclusive formula to fix that. Current models (using BPD, HC, AC, and FL) are the standard but suffer from high observer variability. These formulas struggle at the extremes (macrosomia or growth restriction) and fail to account for fetal adiposity, which makes up 10-12% of a newborn's mass. The study introduces a novel formula that moves beyond simple bone measurements by integrating Soft Tissue Thickness (STT). By accounting for a broader range of biological variants and fat distribution, this new model aims to provide a non-inferior, more precise predictive value for fetal weight compared to traditional methods.

Accurate estimation of fetal weight (EFW) is fundamental to modern obstetric management. Traditional sonographic biometry-utilizing biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL)-is the standard for predictive modeling. However, these parameters are limited by significant inter- and intra-observer variability and a loss of precision at the extremes of fetal weight, such as in cases of macrosomia or growth restriction. Furthermore, conventional formulas often fail to account for fetal adiposity, which constitutes 10-12% of total neonatal body mass. Recent literature suggests that the integration of soft tissue thickness (STT) measurements-specifically mid-arm, subscapular, abdominal subcutaneous, and mid-thigh thickness-can enhance the accuracy of EFW. While the Scioscia and modified Scioscia formulas rely exclusively on FL and mid-thigh measurements, this study proposes a novel, multi-parametric approach. By combining AC, fetal abdominal subcutaneous tissue thickness, FL, and mid-thigh soft tissue thickness, we hypothesize that our formula offers a non-inferior predictive value. This model accounts for a broader range of biological variants and soft tissue distribution patterns that significantly influence overall fetal mass. Detailed history (maternal age, obstetric history, medical history, previous macrosomic fetuses, previous shoulder dystocia). Gestational age is calculated from the first day of the last menstrual period and confirmed by either a first- or second-trimester ultrasound scan. When the ultrasound-determined gestational age differed from that calculated from the last menstrual period by \>7 days in the first trimester, or by \>10 days in the second trimester, the ultrasound-determined gestational age is used. The study will be bi-phasic, 1st phase is model development phase in which we develop a mathematical model for fetal weight estimation incorporating (abdominal circumference, fetal abdominal subcutaneous soft tissue thickness, femur length, mid-thigh soft tissue thickness), the 2nd phase is a validation and comparison phase in which we test our model and compare it to modified Scioscia formula and conventional Hadlock formula. For measurement of femur length (FL) Each caliper is placed at the ends of the ossified diaphysis without including the distal femoral epiphysis if it is visible. For the measurement of AC, the transverse section of the fetal abdomen should be as circular as possible, and the fetal spine preferably in the 3- or 9-o'clock position, umbilical vein at the level of the portal sinus, stomach visible and kidneys not visible. The AC is measured directly at the outer surface of the skin line, with ellipse calipers. Mid-thigh STT: measured linearly in the standard longitudinal section used for FL measurement. In the middle third of the fetal thigh, with the femur lying parallel to the transducer, mid-thigh STT was measured from the outer margin of the skin to the outer margin of the femur shaft. The measurement was taken, providing that the greater and the lesser trochanter are turned upwards. This section assures the correct view of the lateral side of the femur (vastus lateralis, which is the biggest part of the quadriceps femoris) Abdominal Subcutaneous STT: Measured at the level of the AC plane, anterior 1/3rd of abdominal circumference between outer and inner edges of abdominal wall by ultrasound. Actual birth weight (AFW) was immediately measured after delivery by a neonatologist using digital medical neonatal scale.