I'm pregnant with #2 and don't know if she has CF or not. It's hard. I have been able to focus on other things luckily, otherwise the wonder and fear would be eating me alive. She is due in early December so I will let everyone here know what her CF status turns out to be, but in the meantime I am already thinking of a third child but it will depend on this one's CF status.
If you want another child than I fully support you. It is a personal decision, as everyone says, so don't let any fears about what people will say play a part in your decision. At the CF walk last week I wondered what people thought of me all huge and pregnant with a 2 year old with CF. Of course, no one said anything, but I still worry that they judge me. The best part is everyone at clinic is supportive, even the doctors.
So go forward with what decision is best for your family and stand tall in that decision. It is hard to have a kid with CF so I'm guessing it's even harder to have 2 with CF. Tough decision, I know. PM me if you want.