I know when I've been short of breath playing sport, or for singing (I don't sing and play sport at the same time though :P much.) I've been told to move my shoulders forward rather than holding them back. That might create a little more space for your lungs. So maybe leaning forward a little and moving your shoulders forward?

Sorry, last bub engaged at something ridiculous like 29 weeks, so I haven't been in your situation for awhile. Hope you find something that works though!