Well, there may not be a widely known one-off. However, if you dig deeper into the Hetalia fanfiction community, perhaps in some niche forums or groups dedicated to Hetalia fanfiction, you might find some unpublished or less - known works that deal with Belarus being pregnant. You could also try reaching out to other Hetalia fans on social media platforms and ask if they know of any such stories.
Finding a specific 'Hetalia Belarus pregnant' fanfiction can be a bit of a challenge. You could try to think about what kind of story you want within this concept. Do you want it to be a romantic story with a certain character? Or maybe a story more focused on the implications for Belarus herself? Once you have a clearer idea, it can help you better filter through the search results on fanfiction platforms.
A practical answer is Chapter 141, because it turns the idea into a concrete scene. The chapter opens with It had happened in the afternoon when Aunt Ying's son and daughter heard..., adds pressure through Director Sun has already agreed, and the audience in front of the screens..., and ends on.
If you want the clearest chapter reference, go to Chapter 164. Early on, Damn, the production team is so shady, they're still doing a live...; later, the chapter leans on Otherwise, I would never have found out..
If you want the clearest chapter reference, go to Chapter 184. Early on, Jiang Li can finish the competition alone???; later, the chapter leans on Then isn't the championship undoubtedly hers?.
A practical answer is Chapter 177, because it turns the idea into a concrete scene. The chapter opens with I'm actually the last?!" After the scores were announced, Wen Jiaojiao's..., adds pressure through Next time, stay away from me, I get annoyed just by seeing you., and ends on It was with.
The best chapter pointer here is Chapter 144, "She's Too Bullying!!!". It opens with Ordinary people would have been at a loss in this situation by now. and closes on He then noticed the hand—fair and slender, not at all like an auntie's..
If you want the clearest chapter reference, go to Chapter 184. Early on, Jiang Li can finish the competition alone???; later, the chapter leans on Then isn't the championship undoubtedly hers?.
If you want the clearest chapter reference, go to Chapter 151. Early on, Now it was Zhang Cheng's turn to be confused.; later, the chapter leans on The problem lies with He Yun?. By the close, I will have my assistant pay the bill., and the chapter closes with Was he...
A practical answer is Chapter 183, because it turns the idea into a concrete scene. The chapter opens with Seeing that he held a blue ball identical to Su Yinwan's, Jiang Yan..., adds pressure through As for the leftover person—" Saying this, he looked towards Jiang Li, "You..., and ends on Hadn't.
The best chapter pointer here is Chapter 177, "You Annoy Me Just By Being Here". It opens with I'm actually the last?!" After the scores were announced, Wen Jiaojiao's... and closes on It was with great difficulty that she had gained a wave of fans in the....
A practical answer is Chapter 178, because it turns the idea into a concrete scene. The chapter opens with Wen Jiaojiao is just too unreasonable, why does she have to single out..., adds pressure through The gifts were guaranteed to please everyone, and with this, the beloved..., and ends on Let's.