At approximately age 19 years old, trained research staff conducted DXA scans for all participants using the Hologic QDR 4500A DXA (Delphi upgrade) with software V.12.3 in the fan-beam mode, as described previously (4 (link)). Briefly, software-specific Global Regions of Interest (ROI) were used to designate the general boundaries of the hip images. The operator reviewed, edited, and confirmed the bone within the ROI box to ensure appropriate bone-edge detection. The DXA measure used in this study was aBMD (g/cm2) at the total hip. Structural geometry was estimated from hip DXA images using the Hip Structure Analysis program (Hologic Apex 3.0 software). This program is based on the principle first described by Martin and Burr that the mass in a pixel value (g/cm2 of hydroxyapatite) can be converted to linear thickness (cm) by dividing it by the effective mineral density of a fully mineralized bone (31 (link)). A line of pixels traversing the bone axis is thus a projection of the surface area of a bone in cross-section and can yield some of its geometry (2 (link)). Specifically, the Hologic software program located the narrowest point of the femoral neck, where bone cross-sectional area (CSA, cm2) and cross-sectional moment of inertia (cm4) for bending in the image plane were calculated, from which femoral neck section modulus (FN Z, cm3) was derived.