While Peeta is the obvious choice, some fanfictions might explore the idea of Gale being the father. This could add a whole new layer of drama and conflict. Maybe there was a moment of passion between Katniss and Gale before she fully committed to Peeta, and this led to her pregnancy. However, this would be a more controversial take as it goes against the main narrative of Katniss's relationship with Peeta in the original series.
Another possibility could be an original character in the fanfiction. This could introduce a completely new storyline and dynamic. For example, an outsider who comes to their district and Katniss has an unexpected encounter with him. This would lead to a very different kind of fanfiction, with Katniss having to deal with not only the pregnancy but also the complications of having a relationship with someone not from her known world.
A practical answer is Chapter 113, because it turns the idea into a concrete scene. The chapter opens with The next day, Jiang Lili was up before dawn as usual., adds pressure through Okay, keep them steady;
Look to Chapter 81 for the clearest answer. It begins with Qi Tianyu, who chose curry beef rice, received a piece of curry and a..., keeps building through Shu Yan was relatively lucky, she got a bundle of noodles and a bag of..., and lands on The summer on the Southwest Plateau is no joke;
A practical answer is Chapter 97, because it turns the idea into a concrete scene. The chapter opens with Suddenly, Jiang Li's livestream room was packed with people, and the..., adds pressure through Master, you're the only one who can help me now., and ends on And you believe what she says?.
A practical answer is Chapter 57, because it turns the idea into a concrete scene. The chapter opens with Lu Xingchao's numerous fans were stunned., adds pressure through Do you even consider me your brother?, and ends on I'm saying, big bro, can you at least reply to me?.
If you want the clearest chapter reference, go to Chapter 89. Early on, Director Sun took a step back, "What's the rush?; later, the chapter leans on It's true about the go-kart costing one point, but this time we're using....
If you want the clearest chapter reference, go to Chapter 104. Early on, Zhao Lang let out a light chuckle, "Young Master Jiang, don't be nervous,...; later, the chapter leans on You're Daddy's guests, how could I possibly come to settle scores with....
Look to Chapter 77 for the clearest answer. It begins with Wen Jiaojiao's reminder calmed Shu Yan down., keeps building through Indeed, the wild mushrooms on the Southwest Plateau were complex in..., and lands on But who knew that the next second, the lottery box would be handed over to....
Chapter 120 is the strongest chapter-level answer because it starts from Facing the sudden and somewhat strange request from Jiang Li, Director Sun... and follows through on Drink this." Wen Jiaojiao looked at the half cup of thick substance:....
Chapter 117 is the strongest chapter-level answer because it starts from Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi... and follows through on Since entering puberty, his mother had exerted strict control over his....
A practical answer is Chapter 135, because it turns the idea into a concrete scene. The chapter opens with The sun blazed down mercilessly, but thankfully, the path to the mountain..., adds pressure through At this moment, it seemed more likely to be grim than not., and ends on Are you healed from.