I was told when I was pregnant with my son that Cf moms typically go 4-5 weeks early. true to this doc's word, Caleb was born at 34.5 weeks and Marlee at 36 weeks. Both weighed about five and a half pounds and both came right home with me, so trouble needing NICU care (well Marlee went for an hour for observation for retracting, but they think that's because she was born so fast). I gained 25 punds with Caleb and about 20 with Marlee. I had a harder time gaining weight with Marlee, but what I gained, I gained steadily so no one was too concerned.
Both pregnancies required me to go on oral antibiotics once for sinus issues. There was no known reason why I went into labor early - my theory is that once the baby nutritionally demands more than our Cf bodies can give them, out they come just my theory.
as for breastfeeding - breastfeed! Whatever meds you take during pregnancy should be safe for breastfeeding and it is the greatest gift you can give your child. I have lost all my pregnancy weight from bfing, but I don't think I am going to have a problem maintaining a healthy weight from here on out. If I do, I will start supplementing with shakes or something before I stop bfing. While I realize you have to be healthy for your babe to be healthy, bfing is so important. if you can, do it. Even if it looks like you won't be able to breastfeed for a long time, just the colostrum alone is packed with important antibodies for your babe!! This would be one area I would fight my doc on if he was trying to deter you - try it out and see how you do, you can always stop, but once your milk dries up it is very hard to relactate should you decided you want to breastfeed after all.
I will step off my soap box now.