Hi Helena,
Here is my experience.
My FEV1 was stable in the upper 50's when I underwent IVF. Eventually, by the end of pregnancy my FEV1 was higher than it had been in years! (mid sixties) Pregnancy (maybe the increased blood volume, therefore more oxygen???) actually improved my lungs.
After pregnancy I saw my baseline return, minus a few points. I've hovered around 50% FEV1 since I returned to my baseline. My twins are nearly 3 years old now.
In my opinion, the pregnancy brings all sorts of stress onto the body. If you can ward off infection, it's likely your lungs may even improve during pregnancy, as mine did. But if you're already compromised and you get pregnant and then battle infection, you could end up worse that when you started. It's a gamble. There's probably a reason doctors suggest FEV1 above 50% before thinking about becomming pregnant. Honestly, the pregnancy, even with twins, was a breeze compared to the sleepless nights of the first 3 months after the babies got here.
How are you with treatments now? Are you compliant? Do you find yourself in need of IVs once or more per year? Do you consider your health stable at 49%? These are all things to consider before you think about complicating things with a pregnancy.