Your actual chromosomes don’t change over time (with rare exceptions like certain cancers, where testing is done on tumor cells specifically, not blood). So for constitutional/inherited testing like a karyotype result is a one-time, stable answer once it’s finalized. What can change between draws is the clarity or resolution of the result, for the technical reasons discussed above (culture quality, staining, imaging) which is different from the underlying biology changing. If a lab asks for a repeat sample, it’s almost always to get a better technical read, not because your chromosomes themselves are in flux.