Stress fractures are the result of repetitive microtrauma to a bone over time. As this stress is placed on a bone over time, if the bone is weak or unable to recover from this repetitive load it can develop many minute cracks throughout the bone while not breaking it through-and-through. These tiny cracks are not easily identified on x-rays as a typical fracture is because they are so small. When taking an x-ray of a foot that has a suspected stress fracture, there are many other concerns and ailments we are ruling out and after the period of a few weeks we can see signs that a stress fracture is/was present. As the bone heals a stress fracture, it lays down more bone to add strength. Over the course of a few weeks, we can usually see this extra bony growth in the area of a stress fracture. Depending upon where the stress fracture occured, this can be very obvious - but again, it take weeks before an x-ray will demonstrate the bony reparative process.
You may also be interested in this FAQ's