Individualized growth assessment of fetal thigh circumference using three‐dimensional ultrasonography

Objectives To develop individualized growth assessment (IGA) standards for upper (ThC(u)) and middle (ThC(m)) fetal thigh circumferences using three‐dimensional ultrasonography. Methods A prospective, longitudinal sonographic study of 30 fetuses was performed beginning at 18 weeks' menstrual age. Second‐trimester sonographic parameters were measured from three‐dimensional volume data to establish IGA standards. Normal infant growth outcomes were confirmed using modified Neonatal Growth Assessment Scores (m3NGAS51). ThC(u) and ThC(m) were studied in more detail. Rossavik growth model specification procedures, based on the slopes of the second‐trimester growth curves, were developed for both ThC(u) and ThC(m). Third‐trimester growth trajectories and birth measurements were subsequently predicted for these parameters. Percentage deviations during the third trimester and percentage differences at actual birth age were used to compare observed and predicted measurements. The 95% ranges for Growth Potential Realization Index (GPRI) values for both types of thigh circumference were determined. Values for m3NGAS51 using GPRIThC(u), GPRIThC(m) and GPRIThC(o) (original method) were compared. Results The 30 newborns had no postnatal evidence of abnormal growth. Two examiners demonstrated a satisfactory measurement bias of mean ± SD 2.1 ± 3.6 (95% limits of agreement,−4.9 to 9.1)% for ThC(m) and 3.3 ± 4.1 (95% limits of agreement,−4.8 to 11.4)% for ThC(u). Rossavik functions fitted parameter trajectories well, with mean R2 values of 99.5 ± 0.4% for ThC(u) and 99.6 ± 0.3% for ThC(m). By fixing coefficients k at their mean values, their respective fits did not change, and the variabilities of coefficients c and s were significantly reduced. For ThC(u), coefficient c was significantly related to the second‐trimester slope (R2=98.6%), as was s to c(R2=91.0%). For ThC(m), coefficient c was significantly related to the second‐trimester slope (R2=98.6%), as was s to c(R2=85.6%). Third‐trimester growth trajectories, derived from second‐trimester slopes for individual fetuses, had third‐trimester deviations of 0.07 ± 3.7% for ThC(u) and−0.04 ± 3.7% for ThC(m). Percentage differences at birth age were 16.8 ± 10.2% for ThC(u) and 8.9 ± 9.5% for ThC(m). With correction for systematic overestimations, the mean GPRI values were 103.7 (95% range, 90–121)% for ThC(u) and 101.6 (95% range, 88–118)% for ThC(m). Corresponding mean ± SD m3NGAS51 values, using GPRIThC(u), GPRIThC(m) and G