The effects of immobilization on fetal bone development were studied through post-mortem radiographs in 11 newborns with congenital neuromuscular diseases (CNMD) of intrauterine onset. Quantitative parameters were determined in the following bones: tibia, femur, humerus, radius, 3rd lumbar vertebra, and 5th rib. Thirty stillborns or newborns of similar gestational age and deceased from causes other than neuromuscular or related diseases were used as controls. No significant differences in bone lengths were observed between both groups. However, external diameters, cortex thicknesses, and cortical areas values were significantly lower in newborns with CNMD than in the control group. In newborns with CNMD, the medullary diameter of long bones showed good correlation with gestational age. However, this diameter was greater than that expected according to the reduced external diameter. These results suggest that intrauterine immobilization induces a decrease in mechanical usage of bone, mainly influencing bone modeling and probably bone remodeling. No changes were observed in longitudinal bone growth. Bones showed osteopenia and mechanical defects and were prone to fractures. In summary, reduced immobilization in utero produces bone osteoporosis of the fetus.