Some cat owners are concerned because they can feel their cat’s shoulder blades (scapulae). They might feel unsure about their cat’s weight. My cat is neither thin nor fat. When he takes up certain positions such as when crouched down or sitting I can feel his shoulder blades. It is quite normal.
Clearly if a cat is underweight you are more likely to feel her shoulder blades in almost any position. However, feeling a cat’s shoulder blades does not automatically mean the cat is thin. It is a question of the cat’s stance and positioning.
Below is a picture of a cat of normal weight and you can see (and therefore feel) her shoulder blades. I think that supports the point I am making.
A simply test to check your cat’s weight is to see if you can feel her ribs easily. If not your cat is almost certainly overweight. If the ribs are too pronounced she’s likely to be underweight. I have to say it is largely a matter of common sense deciding if your cat is under or overweight. And don’t worry about being able to feel your cat’s ribs unless she is underweight. If in doubt see your veterinarian. Many serious diseases cause loss of weight due to loss of appetite.