There could be a theme of secrecy. Given their status and the nature of their relationship, they would probably need to keep the pregnancy a secret from many, which could lead to all sorts of plot twists and internal conflicts within the story.
I'm not sure of specific 'bellamort pregnant fanfiction' titles off the top of my head. But you can try searching on fanfiction websites like Archive of Our Own. They have a wide range of fanfictions, and you can use the search filters to look for stories related to Bellamort and pregnancy.
One common theme is survival. Since 'The 100' is set in a harsh world, the pregnancy adds another layer of challenge. The characters have to ensure the survival of the mother and the unborn child in a world full of danger.
Many Bellamort fanfictions play with the idea of redemption for them. Since they are villains in the original story, some fanfictions might explore a scenario where they find a way to change their ways and how that would affect their relationship.
Love and support are common themes. Usually, Bella's relationship with Edward or other important characters deepens as they support her through the pregnancy.
Another theme is the relationship changes. Her relationship with the baby's father might be explored, whether it's a new relationship or an existing one. There could also be changes in her friendships as she focuses more on her upcoming motherhood.
Family support is also a big theme. It shows how their friends and family react to the news, whether it's with joy, concern, or a mix of both. In some garmau pregnant fanfiction, the family members play an important role in helping the couple prepare for the baby.
One common theme could be the relationship changes. With the pregnancy, the characters' dynamics might shift in many ways.
One common theme could be the challenges of pregnancy. Since Undyne is a strong character, it might be interesting to see how she copes with the physical and emotional changes that come with pregnancy. Another theme could be the relationship between Undyne and her partner during this time, like how they support each other.
One common theme might be the anticipation and excitement within the family and the community. Since Neytiri is an important character in the Avatar universe, her pregnancy would likely have a big impact on those around her.
One common theme could be the exploration of the unexpected situation of a character like Chucky being involved in a pregnancy - related plot. It might also deal with themes of the supernatural and how it intersects with pregnancy.