Common themes in such fanfiction can vary widely. There could be the theme of sacrifice. For instance, Rogue might have to sacrifice some of her freedom or her usual activities to ensure a safe pregnancy. Another theme could be the exploration of their characters' insecurities. Gambit might be insecure about being a good father due to his past, and Rogue could be worried about her ability to be a mother while having her powers. The theme of family, not just their new family but also how their relationship with the X - Men as a family - like group changes during the pregnancy, is also often explored.
One common theme could be the challenges they face as a couple with the pregnancy. It might include things like Rogue's powers and how they could potentially affect the pregnancy or Gambit's past coming back to haunt them during this new phase of their relationship.
A theme that often shows up is the anticipation and excitement of starting a family. In 'rogue and gambit pregnant fanfiction', there could be scenes where they are preparing for the baby, like decorating a nursery or choosing names. Also, the theme of their love growing stronger through this new experience is quite common. They might have to rely on each other more and face their fears together. Another possible theme is how the X - Men community reacts to their pregnancy, whether they are supportive or if there are some concerns within the group.