Missing a milestone in the path of the project is considerd as a failure. However, that is not the end of the project or that doesnt mean that there is any compromise in the project in regard with its quality/reliability/safety/durability. There should be another plan ready to be executed incase of failure. If the delay for the milestone to be reached is reasonably justifying that it has something to do with patient safety and the process to accomplish it is more challenging then probably it might be considered. This would also give space for learning opportunity and room for conducting more risk analysis.