It could be. There are many different types of fanfiction themes and pregnancy is a common plot device in some fanfic communities. However, its popularity would depend on the specific fandom it pertains to.
One possible plot development could be that they face challenges in their relationship due to the unexpected pregnancy. Maybe there are family or social pressures.
One challenge could be the acceptance from the two different communities, the werewolves and the vampires. Another might be the safety of Bella during the pregnancy, as it could attract unwanted attention. And of course, there's the relationship between Paul and Bella, which may face strain due to the sudden change in their situation.
Well, often the story develops with Bella having to deal with the new reality. She may face rejection from some in the vampire world who were part of her life before. The pregnancy could be a difficult one, with possible threats to her life. Paul would be constantly by her side, trying to find ways to keep her safe. And there could be a power struggle within the werewolf pack regarding this unexpected situation.
One possible fanfiction could start like this: Bella was shocked when she found out she was pregnant. Paul, on the other hand, was a mix of emotions - excited yet worried. He held Bella's hand tightly as they sat in the doctor's office, staring at the ultrasound picture. They knew their lives were about to change forever. They started to plan for the baby's arrival, from painting the nursery to picking out names. Bella was determined to be a great mother, and Paul was ready to support her every step of the way.
For Paul, you could show his development from a carefree person to a more responsible one. He starts to think about the future, gets a job if he didn't have one before, and becomes more protective of Bella. For Bella, she might become more confident in her decisions. At the start, she could be a bit hesitant about the pregnancy, but as time goes on, she embraces it and starts making plans. You can also show their relationship development. They start to communicate better, understand each other's needs more deeply, and grow closer as they anticipate the arrival of their baby.
Well, a possible plot for a Paul and Bella pregnant fanfiction could involve them going on a journey to find a safe place for the baby. Since they might be in a world full of danger or just a place where they don't feel completely secure. Along the way, they meet new friends who help them and teach them things about parenting. Or perhaps, the pregnancy is a result of some kind of magic or fate, and they have to figure out what that means for their future and the future of their child. They could also face a villain who wants to harm the baby for some reason.
One challenge could be the reactions of their families. If their families didn't approve of their relationship, the pregnancy might cause more tension. Another challenge could be financial. Preparing for a baby is expensive, and they might struggle to afford all the necessary things like diapers, clothes, and a crib. Also, Bella's health during the pregnancy could be a concern. There could be complications that require special care.
Well, it could be that Bella decides to raise the baby on her own without Paul. She could focus on building a support network of friends and family. Another aspect could be that the werewolf or vampire community (depending on the context of the original story) has some sort of reaction to Bella's situation. For example, they might try to protect her from any threats that come her way because of the unborn child.
Another theme is the idea of Bella's self - discovery. Pregnancy is a big life change, and in these fanfictions, she often discovers new aspects of herself. For example, she might find a new strength she didn't know she had, or she might face insecurities she has to overcome.