Please try not to worry - hard as it is.

The Dr might be worried about some conditions which cause the lower jaw to be small - maybe that's what they want to double check. I wouldn't go back looking at the internet on this again until after the next scan - you'll just frighten yourself as there are a ton of 'possibles' and you may get worked up for no good reason. Just take deep breaths, wait for the ultrasound, ask specific questions and get the Dr to write the answers down for you (if there are medical terms, you want to make sure you get them right!).

Wishing you luck on Tuesday.