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.
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.
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.
Chapter 99 is the strongest chapter-level answer because it starts from Seeing the man approach, one of the individuals with a long scar across... and follows through on Just as she was considering whether to practice a set of boxing, her phone....
Chapter 109 is the strongest chapter-level answer because it starts from And another thing," Jiang Li said, taking out a tissue to wipe her lips,... and follows through on [You're indeed more likable than me, the type of likable where people....
If you want the clearest chapter reference, go to Chapter 90. Early on, Watching He Yun gradually take control of a tractor under professional...; later, the chapter leans on Director Sun," Qi Tianyu, having finished admiring, started to complain,....
A practical answer is Chapter 109, because it turns the idea into a concrete scene. The chapter opens with And another thing," Jiang Li said, taking out a tissue to wipe her lips,..., adds pressure through Get them to find him, no matter the cost, I want to ensure Jiang Yan can't..., and ends on.
The best chapter pointer here is Chapter 123, "Can This Even Be Calculated?!". It opens with After He Yun also carefully rolled up her trouser legs and walked down... and closes on Seeing this, He Yun chuckled softly to himself, then glanced at his own....
Chapter 87 is the strongest chapter-level answer because it starts from Luckily, at that time, Jiang Li had already taken her pot of stinky... and follows through on I'm driven crazy by him every day.] [+1 from above!.
If you want the clearest chapter reference, go to Chapter 88. Early on, She wanted to refuse, but didn't have the courage to do so.; later, the chapter leans on The Chen'an Entertainment she worked for was currently the largest....
Chapter 118 is the strongest chapter-level answer because it starts from At 8 in the morning, several guests appeared on time next to the... and follows through on Hearing this, both Jiang Yan and Shu Yan started to complain as well..
The best chapter pointer here is Chapter 100, "Ominous, there might be a bloody disaster!". It opens with The barrage was about to die of laughter. and closes on Her eyes flickered slightly as she grabbed the hand..