Tetras don't get 'pregnant', she could be full of eggs, in which case there's nothing you ned do; tetras are not a very easy fish to breed for beginners.
Or she could have a bacterial infection causing dropsy, where the fish swells up. Are her scales sticking out, giving her a 'pinecone' like appearance? Have you tested your water? How long has the tank been set up and what other fish are in there? Is the fish acting normally and eating well?