P197: Fractional thigh volume: a new soft tissue parameter for fetal growth assessment

Abstract

Previous attempts to quantify fetal soft tissue by conventional sonography have been difficult. Fractional thigh volume measurements, obtained by three-dimensional ultrasonography (3DUS) were evaluated for this purpose, using Individualized Growth Assessment (IGA). Longitudinal sonographic studies of normal fetuses were performed to obtain standard biometry and fractional thigh volumes. Fractional thigh volumes were based on 50 percent of the femoral diaphysis length. Start points, the theoretical age at which an anatomical parameter can first be identified, were calculated from linear functions relating the cube root of fractional limb volume to menstrual age prior to 28.5 weeks'. Regression analysis was used to fit Rossavik functions (P = c(t) k + s(t), where P is the anatomic parameter, t is menstrual minus start point, and c, k, s are model coefficients) to individual growth trajectory data. Rossavik models specified from the linear slopes of growth curves before 28.5 weeks were used to predict TVol during the third trimester and at birth. Neonatal growth status was determined from anatomic measurements and 3DUS within 48 hours of delivery. 22 fetuses (14 females, 8 males) were prospectively studied between 17.4 and 39.3 weeks, menstrual age. The mean birthweight was 3296 ± 253 (SD) grams. All neonates had normal modified Neonatal Growth Assessment Scores. The mean start point (9.0 ± 1.4 weeks') was consistent with completion of thigh development at 8.5 weeks', determined from embryological studies. Rossavik models fit the TVol trajectories very well (mean R2 = 99.8 ± 0.2). By fixing the k coefficient at its mean (2.976), the fit did not change and the variabilities of coefficients c and s were reduced. The mean percent deviation between observed and predicted TVol measurements was − 0.048 ± 7.5% (n = 72) during the third trimester. Neonatal TVol measurements in 12 infants, corrected for systematic error (− 16.7%) led to an averaged signed deviation of − 1.8 ± 9.2% from predicted values. IGA standards for fractional thigh volume provide a new approach for characterizing soft tissue growth. Normal growth trajectories and neonatal soft tissue volume can be accurately predicted from growth models specified by fractional thigh volume data obtained before 28.5 menstrual weeks. The normal variability for these predicted TVol measurements is about one-third of what has been reported for fetal thigh circumference. This parameter may allow earlier detection and improved monitoring of fetuses with soft tissue abnormalities such as IUGR and macrosomia.