Just don't do it. Your girlfriend should actually find it hot that she turns you on that much. And you can't do much about it anyway, because to stop it you would need to focus and your girlfriend...
It should happen if your relationship works well
You keep kissing pationatly until she looks. Give it some tongue to! maybe put your hands through her hair.
It will actually accelerate her due date ( the kissin ) but other than that your showing your kid affection i quess.