Well, it could involve themes of friendship turning into a deeper bond. Since Harry and Hermione have a strong friendship in the original series, in these fanfictions, it might show how that friendship evolves into a relationship where they are having a child together. It might also touch on the challenges they face in the wizarding world as a couple expecting a baby, like dealing with magical threats while also preparing for parenthood.
One common theme could be the exploration of their relationship in a more domestic or family - oriented way. Even without the inappropriate aspects, it might focus on how they support each other during the pregnancy. Another theme could be the reaction of their friends in the wizarding world to this new development in their lives.