Determine the value of the number ABCD

A boy had reached four without giving up the habit of sucking his thumb, though his mother had tried everything from bribery to reasoning to painting it with lemon juice to discourage the habit. Finally she tried threats, warning her son that, "If you don't stop sucking your thumb, your stomach is going to blow up like a balloon." Later that day, walking in the park, mother and son saw a pregnant woman sitting on a bench. The four-year-old considered her gravely for a minute, then spoke to her saying, "Uh-oh ... I know what you've been doing."