I know why pregnant women waddle - because pregnancy is uncomfortable! When Joanna decides to nestle herself as low as possible, it's difficult to walk like a normal person. Sometimes doing a crab-like jig is far less painful.
On a side note, I have no idea how Victoria Beckham does it - she is baking her fourth baby and wears heels everyday!! (these are 5 inches!)
So when you see a prego waddle, please don't laugh. Don't point out how silly she looks when she waddles. Sure it's funny looking and you're welcome to laugh in your head, but laughing out loud means you're risking your life.