Hi there,
1) For me, my PFT's did not decrease during pregnancy. In fact, they actually went up a little, higher than ever, during my 2nd trimester. I did have an exacerbation at 4 months pregnant (not sure what my PFTs were), but I took Colistin and it took care of it. Even though my PFT's were higher, I did have some problems with mucus in my lower lobes due to the baby pushing up and not being able to take deep breaths. But overall, no problems with PFT's.
2) My PFT's did drop significantly after having the baby. I went from 95% during pregnancy to 60% 3 weeks post-partum. My doctor knew this was just due to being post-partum, so he was not too concerned; although, he did tell me that some CF women do not regain all of their lung functions that they lose post-partum. I was completly EXHAUSTED after I had the baby (no sleep, in pain, lost weight, etc). On top of being post-partum, the baby was colicky, so we were miserable for the first 6 months. My CF doctor just wanted to wait it out and see how they increased after the post-partum period. So after a couple months, my PFT's did increase significantly and pretty much back to baseline. I know for sure it was just due to exhaustion because I didn't even have the energy to blow.
Hope that helps. If you have any more question, feel free to PM me.