Fractional limb volume--a soft tissue parameter of fetal body composition: validation, technical considerations and normal ranges during pregnancy.

Abstract

The main goals were to provide normal reference ranges for fractional limb volume as a new index of generalized fetal nutritional status, to evaluate the reproducibility of fractional fetal limb volume measurements during the second and third trimesters of pregnancy, and to demonstrate technical considerations for this technique. This was a prospective, cross-sectional study of gravid women during mid to late pregnancy. Fractional limb volumes were based on either 50% of humeral or femoral diaphysis length. Each partial volume was subdivided into five equidistant slices that were centered along the mid-arm or mid-thigh. Slices were traced manually to obtain fractional arm (AVol) or fractional thigh (TVol) volume. Reproducibility studies were performed, using Bland-Altman plots, to assess blinded interobserver and intraobserver measurement bias and agreement. Selected images were chosen to demonstrate technical factors for the acquisition and analysis of these parameters. Reference charts were established to describe normal ranges for AVol and TVol. Three hundred and eighty-seven subjects were scanned to include 380 AVol (range, 1.1-68.3 mL) and 378 TVol (range 2.0-163.2 mL) measurements between 18.0 and 42.1 weeks' menstrual age. No gender differences were found in these soft tissue measurements (AVol, P = 0.90; TVol, P = 0.91; Mann-Whitney test). Intraobserver mean bias +/- SD and 95% limits of agreement (LOA) for fractional limb volumes were: 2.2 +/- 4.2% (95% LOA, - 6.0 to 10.5%) for AVol and 2.0 +/- 4.2% (95% LOA, - 6.3 to 10.3%) for TVol. Interobserver bias and agreement were - 1.9 +/- 4.9% (95% LOA, - 11.6 to 7.8%) for AVol and - 2.0 +/- 5.4% (95% LOA, - 12.5 to 8.6%) for TVol. Technical factors were related to image optimization, transducer pressure, fetal movement, soft tissue compression and amniotic fluid volume. Fractional limb volume assessment may improve the detection and monitoring of malnourished fetuses because this soft tissue parameter can be obtained quickly and reproducibly during mid to late pregnancy. Careful attention should be placed on technical factors that can potentially affect optimal acquisition and analysis of these volume measurements.