Pregnancy discrimination in the workplace is illegal, but it happens all the time. So you need to be proactive. It’s not as if your company’s gonna grow a heart, all of a sudden.
First, put it in writing. Let them know as soon as you are ready to announce to the world. Let them know that you very happy, that you’re pregnant, and find out what this company’s pregnancy/maternity leave policies are, because it’s coming up six months from now. Then, if your company starts giving you a hard time in any point during the pregnancy, put that in writing too, and call it what it is: Pregnancy Discrimination.
This way, if your company doesn’t do what they must do, and be flexible and accommodate you, and not treat you differently because of your pregnancy – if they don’t fix it, you will be able to fix their wagon, on your way out the door.