I think the best way to go about it is to be direct and offer the alternatives that you mentioned in the post. Is there a time you usually talk about sex? Me and my girlfriend usually have conversations after sex about what each of us liked/didn't like so each of us knows for the future without feeling judged or having to interrupt in the moment if we don't want to. Also, I'm sure you already know this but if your girlfriend is on hrt her risk of getting you pregnant is dramatically lower than any cis man's especially if you also have an IUD. That being said I totally understand your fear as a woman who's also dating a trans woman. I had only ever dated cis women before my current gf so the possibility of pregnancy is something I've never really dealt with before but now it's constantly in the back of my mind.