Dress up as a imprisoned pirate - modify your costume to have your belly be the 'ball' of a ball and chain - that you have to carry around everywhere you want to go. Attach a plastic chain from the 'ball' down to your ankle.

Pirate with cannon ball - replace chain above with fuse

Pirate with keg - (I would imagine this would be a bit more difficult to accomplish)

Pirate with head of the last man who called you a wench hanging from her belt (yes, that guy had a big head!)

You could use a fake arm holding each of these (yes, you just happen to be a three-armed pirate! - of course, at nine months pregnant, one hand is almost always at the small of your back, isn't it...)

Other options: Pregnant non-pirate (nun, rich governor's wife, etc...) captured by loathsome pirates....