If you find your kid's height is below or far below the bottom line or her/his trend of growth, please keep monitoring as this can be indication of health problem.
This is the case to my kid who has been luckily found of genetic problem and needs to take different tests and treatment.
However, don't be panic when you find out the progress is not good. Please refer to the last attached web on 18/3/2011 and check out if anything can be done.