Show a community of support around her. In the fanfic, she could be part of a group of friends or a family that loves and accepts her unconditionally. They celebrate her body changes, and through their positive influence, she learns to love herself more. This could involve scenes where they go shopping for new clothes that flatter her new shape, or they engage in activities that make her feel good about herself regardless of her weight gain.
Highlight the beauty in all body types. Describe how the female character's new shape, with the belly weight gain, is just as alluring and unique as any other. Use vivid language to show that her body is a work of art, and that her worth isn't defined by her size. For example, you can write about how the curves of her belly are like gentle waves, adding a new kind of charm to her overall look. Also, make sure to give her a strong and positive inner voice that embraces her body and encourages others to do the same.