By careful observation, the moment at which the rib-cage flattens can actually be seen to occur. This happens between 2-10 days from birth, and by feeling the 'walls of the chest' with your fingers you can often detect the flattening before you can see it by finding a ridge running longways at the sides of the ribcage.
A reasonably fresh banana.
It's very difficult to explain what a flat chest feels like in words, but if you can get hold of a reasonably fresh banana (yes, a banana) you can feel what a flat ribcage is like: I've cut this one in half so that you can see the shape in cross-section. Hold the banana with the flattest side downwards (usually on the inside of the curve). Hold it as I've illustrated, and draw your hand upwards. You can see that the outside of the skin has two clear ridges at the lower edge, and the bottom is flatter than the sides and top. This is almost exactly what a mildly flat chest feels like (even down to the size) only with a covering of fur.
You may feel a slight ridge running along the ribcage in some kittens that does not progress into FCK, so don't panic!
Normally the flat chest is simply that: flat. But if the muscles of the diaphragm are severely affected they will pull the sternum into an abnormal position -- i.e. the sternum will be pulled into the chest cavity, and this in turn will distort the shape of the heart and prevent the lung tissue from developing normally. This pulling in is one reason the condition is sometimes called pectus excavatum, but usually the sternum does not grow inwards, it is pulled in with the ribcage. When this happens recovery is unusual.
There is another condition (unrelated to FCK) in which the sternum sticks out, and though this apparently has no health implications, it is considered a veterinary fault.
Flat chested kitten
This kitten (right) has an inverted sternum - you can clearly see it is dipped into the body instead of curving outward.
The kitten was a single FCK in a healthy litter, and was losing weight and clearly failing until he was put onto supplement feeding with KMR. He is now happy and healthy, though his sternum has not righted itself. He is smaller than his siblings.
(Many thanks to his owner for sending his picture and regular updates on his progress.)
[If you have photographs that show FCK it may be useful to other people to be able to see them - I would like to put them here if they show the condition. E-mail me!]
There are degrees of flat-chestedness -- sometimes occurring in the same litter. Many slightly flat-chested kittens go unnoticed and recover on their own. Various causes have been put forward for this condition developing: The Burmese Cat Club funded research into Taurine deficiency which, though it only had a very small study group to work with, concluded that it was not relevant. I have scanned the pages, and images of the article can be downloaded here (click on the filenmames on the list to download them). I apologise if this infringes any copyright, but the article was printed some years ago and is difficult to obtain now, and I feel that providing information to save lives must outweigh copyright concerns. Other theories have suggested Potassium deficiency, but treatment with potassium supplements as suggested by Bristol Veterinary College have not shown significant results in treating the condition, and blood samples have not suggested a deficiency. There has been no long-term or serious study into this condition, and for this reason I am assembling data for the future so that if such a study is undertaken a sizeable sample of litters can be studied to gain more information.
Rex kitten with FCK - note also the dip in the spine just below the shoulders.
Egyptian Mau with FCK - note splayed elbows pushed out by the sides of the ribcage.