Well, I'm somehow confused. How come お腹 is pronounced as おなか but 腹 as はら?
Moreover, 腹 doesn't even have なか reading in dictionary.

お腹 was 御中 earlier. The latter compound still appears in dictionaries, but the more popular version now is お腹, with kanji switched so the meaning is clearer. The difference is that お腹 is polite and 腹 （はら） is not.

Thanks, Rayath!