Well, in a Nina is pregnant fanfiction, the story could revolve around the reactions of her friends and family. Some might be overjoyed, while others could be worried or judgmental. For example, her conservative parents might have a hard time accepting the situation at first. Also, the story could involve Nina preparing for the baby, like decorating the nursery. She could be really into DIY and spend hours making the perfect little room for her soon - to - be - born child.
A possible story could be that Nina is pregnant in a world where pregnancy is rare or has some special significance. This could lead to her being a sort of figure of hope or fear depending on how the society views it. There could be a mystery element, like not knowing who the father is at first. Or perhaps Nina has some sort of special power that is affected by her pregnancy, and she has to learn to control it while also dealing with the normal pregnancy stuff.
One possible storyline could be about Nina's journey through her pregnancy. It might focus on how she copes with the physical changes, like morning sickness and fatigue. Maybe she has a supportive partner who is always by her side, taking her to doctor appointments and helping her with household chores. Another could be that Nina discovers she's pregnant at an unexpected time, like when she's in the middle of achieving a big career goal. This creates a conflict between her personal and professional life.