Don't stress out about this, just prepare adequately by studying your material from your class and practicing.
Driving is driving; the same routine no matter whether you are pregnant or not!

Make sure your seat is comfortably positioned where your belly is not too close to the wheel, and also be sure that you can comfortably reach the floor pedals. The only other advice I can offer is don't miss your breakfast that day because you are anxious; low blood sugar and lightheadedness could definitely affect your performance on the test.
Good luck on the test and best wishes as you soon welcome your baby!