By what mechanism can two chromosomes fuse?
Chromosome fusions (as well as fissions and other changes) usually take place during cell division or pairing, therefore, it's very unlikely that two chromosomes will fuse in an already fertilized egg. If they do such, this egg would probably be aborted. 

One factor that may lead to chromosome fusion is loss of thelomeric sequences (telomere = chromosome extremity) (O'Sullivan and Karsleder, 2010) - but probably not in an fertilized egg.