Congratulations on your pregnancy, but I’m sorry for your worry about your health. It’s been awhile since I’ve been through this as my youngest is 13 years old, but I do remember that I had a decrease in PFTs and increase in symptoms with my second pregnancy. I ended up on a course of IVs in my second trimester, I think around the 4th-5th month. I remember being so worried about the effects of the antibiotics on the baby, but the benefits outweighed the risks. My daughter is fine, without any negative effects from any of my meds.
Not sure if antibiotics would be the answer for you if you’re not having an increase in other symptoms. As the pregnancy progresses, the baby can start pressing up on your diaphragm and decreasing your lung capacity, so maybe that’s what’s going on. I had extra amniotic fluid with second pregnancy, so I always measured larger than expected and was much more SOB with my second pregnancy than my first. SOB immediately resolved with delivery.
There is a Facebook group for “CF Mummies” that is a private group that you might be interested in joining. You’ll probably get a lot of feedback there. I’m not on Facebook, so I’m not a member of the group but have heard positive things about it. I’ll PM another member of this forum to provide the link. Hopefully it’s still an active group. I’ll post if I find it out.
Best of luck to you.