Our son was born November 6, so by the time Halloween rolled around, it absolutely felt like he was trying to burst out of her belly like the original "Alien" critters. I thought, "he probably just wants some candy!", and so this costume was born.
Making it couldn't have been simpler. I used a stretchy, spandex-like maternity tank she already had on hand, an appropriately sized baby doll from the thrift store (the kind with plastic arms attached to stuffed fabric body) and some fake blood from the Halloween store. I cut two holes where I wanted the arms to protrude, making them much smaller than the diameter of the arms so the fabric would have to stretch to accommodate them, thereby holding them in place. I placed the arms, then sprayed some fake blood around the exit points. Once my wife put the shirt on, the tension of it stretched over her belly kept the arms pointed outward. I added a tiny jack-o-lantern candy bucket (he was wanting some candy after all), and it was complete!
You could tell as she was handing out the candy that some people thought she was REALLY sick. People either laughed out loud, or were unable to hide their disgust. Either way, it was AWESOME.