The Fetal Arm

Objective The goals were to introduce fractional arm volume (AVol) as a new soft tissue parameter of fetal growth assessment and to develop individualized growth standards, based on Rossavik models, for AVol, midarm circumference (ArmC), and humeral diaphysis length (HDL). Methods A prospective longitudinal study of 22 fetuses was conducted using 2‐ and 3‐dimensional sonography. Three new growth parameters (HDL, ArmC, and AVol) were used to establish individualized standards for arm growth with the use of Rossavik functions [ P = c ( t ) k + s ( t ) , where P is the anatomic parameter; c , k , and s are model coefficients; and t is the time variable]. Second‐trimester models were specified from the linear slopes of growth curves before approximately 28.0 menstrual weeks. For a given fetus, normal third‐trimester trajectories were predicted for each parameter. Observed and predicted measurements were compared by percent deviations. Results Rossavik functions fit all parameter trajectories extremely well ( R 2 = 95.7%–99.4%). By fixing coefficients k at their mean values, their respective fits did not change, and the variabilities of both coefficients c and s were reduced. Coefficient c was also significantly related to second‐trimester slope, as was s to c , for all 3 parameters ( R 2 = 97.7%–98.7%; P < .0001). Mean percent deviations between observed and predicted third‐trimester HDL, ArmC, and AVol measurements were −0.1% ± 2.9%, 0.5% ± 4.6%, and 0.4% ± 8.5%, respectively. Conclusions Individualized growth assessment, using HDL and ArmC, can accurately predict normal arm growth during the third trimester of pregnancy. AVol may also allow earlier detection and improved monitoring of soft tissue abnormalities that can occur in fetuses with growth disturbances.