I'm going to ask my pregnant gf for a dna test
Why do you think it's necessary?
I don't know if the baby is mine.
If you do this she might never trust you again and destroy your relationship with her and the child. People are expected to trust their partners.
I do trust her, but I want to make sure.
I know you want to make sure the child is yours, but in this way it seems like you are accusing her of cheating. There needs to be more trust in the relationship.