The aim of this study was to investigate the relation between birth weight and calcaneal bone stiffness in a large sample of Belgian, healthy, preadolescent children. Participants were 827 children (3.6-11.2 years, 51.6 % boys) from the Belgian cohort of the IDEFICS study. Birth weight was obtained using a parental questionnaire, and quantitative ultrasound (QUS) measurements were performed to determine calcaneal broadband ultrasound attenuation (BUA), speed of sound (SOS), and stiffness index (SI) using the Lunar Achilles device. Average birth weights were 3435.7 ± 512.0 g for boys and 3256.9 ± 471.1 g for girls. Average calcaneal QUS measurements were 89.6 ± 24.0 (23.3-153.9) dB/MHz for BUA, 1621.4 ± 49.6 (1516.3-1776.5) m/s for SOS, and 92.8 ± 15.6 (49.0-163.0) for SI. Birth weight was positively associated with BUA (r = 0.13, p = 0.002) and SOS (r = -0.16, p < 0.001). The associations remained after correcting for age and sex in multiple regression analyses but disappeared after correcting for anthropometric covariates. Our findings suggest that birth weight, as a rough proxy indicator for genetic and environmental influences during intrauterine life, is associated with BUA and SOS in preadolescent children and may therefore influence the risk of osteoporosis later in life. Further studies using QUS are needed to investigate the consistency of the results of this study.