More often than not these swallowed fingernails get digested in the strongly acidic conditions of the stomach and don't cause any harm. But in some cases they accumulate in the gut and over time start causing intense abdominal pain that has to be surgically operated to remove it. read more
However, in some cases, things that are undigestable such as watermelon seeds, fingernails, etc can get caught in a diverticuli in the intesting and become coated with food, causing it to get bigger and bigger. read more