Flat chest kitten may be more prevalent in dwarf cats than for other breeds due to the dwarfism gene.
Sometimes kittens in a litter thrive less than other kittens. This, of course, is normal. However, in a litter of dwarf cats (e.g. Munchkins, the founding breed of dwarf cats), the “Standards” (as per the standard desired – short legged kittens or dwarf kittens) thrive less well and are more vulnerable than the “Non-standards” (kittens of the same litter from dwarf cat parents who are born with normal length legs).
This may be due to a number of causes such as illness, infection, secondary infection etc. A vet of course will advise.
However, as I have mentioned elsewhere, not enough is known about dwarfism in cats. I have written about health issues of dwarf cats generally.
Breeders, however, report that one (possibly not too uncommon) cause is that the kitten is unable to breath properly due to a condition known as Flat Chest Kitten or FCK. [Note: buyers should check for this and ask questions in my opinion]
The condition is not apparent at birth. It becomes more obvious after a few weeks. If slightly affected the kitten appears normal. If moderately to badly affected the kitten will have difficulty breathing and will suffer poor weight gain. The worst affected kittens will die.
FCK is not pectus excavatum chest, which I discuss as well.
A check can be carried out by the breeder or buyer. A tight chest feels different.
A normal chest is somewhat rounded 0. You can see the comparison between a normal chest and pectus excavatum chest here.
The FCK chest