Pregnant fiance has borderline personality disroder. will this affect the baby in any way?

Sadly, it might: I imagine you're both being optimistic, i even feel the urge. It's possible that, with love, care, support & patience things may go well. Everyone will be able to feel a true sense of pride and love for themselves and each other. Watch out for any acting out behaviors - both high & low - both during pregnancy and after birth. Be sure all dr.S are aware. Remind her that it is not an issue of trust.