I took a class at the Y today - "Pure Strength" - which was more entertaining than I expected. Out of the hope I'd get cues for modification, I told the instructor that I was pregnant and that I may not be able to do everything. "Sure you can - we won't be laying on our stomachs." That made me giggle. She walked away before I could get out that I'm not supposed to lay flat on my back.
Anyway - I aerobicized my little heart out for the hour, self-modifying when needed, and came out comfortably sore.
Of course, I tore into lunch like a vulture at a buffet, but that's an entirely different workout story.