Well, I'm not sure specifically which 'Jon Snow Sansa Stark pregnant' fanfiction is the best. But often in these types of fanfictions, authors might take creative liberties. They could build on the existing relationship between Jon and Sansa from the show or books. Maybe they'll write about how the two deal with the challenges of a pregnancy in a world full of political intrigue and power struggles. You might also find some that incorporate other characters' reactions and how it affects the overall story arc in their fictional world.
Yes, there probably is. Fanfiction communities are quite vast. However, the quality can vary greatly. Some may focus more on the relationship between Jon and Sansa leading up to the pregnancy, while others might explore how their family and the Seven Kingdoms react to the news. It really depends on what you're looking for in a fanfiction.
There are likely many fanfictions on this topic. You can start by checking popular fanfiction websites like Archive of Our Own. There, you can search for the specific tags 'Jon Snow' and 'Sansa Stark' along with 'pregnant' to find relevant stories. Some might be really well - written with interesting plot twists and character developments.