As an adopted person "biological <insert title>" has worked well for the parents that had sex to make me. For a donor parent, I probably just use "donor <insert title>". I'd advise not worrying too much about the language though. Being kind and thoughtful is far more important than selecting the correct words. A snap judgment selection of proximal words is sloppy but it's impractical to pause to select exactly the right language in all cases for all statements. So too with something this sensitive it might be good to slow a little.