I feel like there should be an exception for pregnancy. At my work, in positions that require tucked in shirts, you don't have to when pregnant as long as you are wearing a tailored shirt (not just a huge baggy one).
I'm only 30 weeks and already tucking in a shirt would be less than attractive, let alone professional looking. If your job requires a tucked in shirt because of safety reasons (like in a factory) then I would just tuck into regular maternity pants and look kind of goofy but be safe, but if it's required because they want you to look professional, I would have a sit down with my boss to discuss an exception.
Re: Maternity pants at work.