Dress code policy is to wear our shirts 'tucked in'. I am pregnant, so do I have to tuck in my shirt?

My suggestion is that you address the issue with HR and be ready to explain how uncomfortable and impractical this would be, especially as you get closer to your due date.  You are entitled to certain possible accommodations, but you need to differentiate between dress codes and clothing styles and the exception that should exist for pregnant women when you raise this issue to HR.