I'm not sure specifically about a 'George pregnant with Fred baby' fanfiction. It's a rather unusual concept and might not be very common. You could try searching on major fanfiction websites like Archive of Our Own or FanFiction.net.
There are likely some out there. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'Fred and George pregnant' in their search bars and browse through the results. Some might be really well - written and capture the essence of the characters in an interesting pregnancy - related storyline.
One common theme could be the shock and adjustment of becoming fathers. Another might be the support they get from their family and friends. Also, the anticipation and excitement of the new arrival.
It would be a very strange and unique take. It might involve some sort of magical or alternate - universe scenario to make it possible. Maybe in a world where gender roles and biological functions are different in the context of that fictional universe.
Hogwarts: Weasley Is Our King was a recommended novel, which included supporting actors Fred Weasley and George Weasley. This book can be read on Qidian Chinese website.
One reason could be that they played a particularly mischievous prank on her that went too far. For example, if they made her late for an important exam or meeting with a professor by using one of their wild magical inventions. Hermione, being a stickler for rules and academic achievement, might lose her temper and hex them in retaliation.
A characteristic is the playfulness. Since Fred and George are known for their pranks in the original series, this often carries over into the fanfiction. So, the kiss might happen in a really unexpected and fun situation. Like, they might be in the middle of a prank gone wrong or right and then suddenly share a kiss. Also, the fanfiction usually explores their individual personalities within the context of their relationship. For instance, Fred might be more impulsive when it comes to expressing his feelings, while George is more reserved. The kiss then becomes a way to show how they navigate these differences.
In some fanfictions, they could be seen as soulmates in a platonic sense. Their strong bonds of friendship and the adventures they share might be interpreted as a form of soulmate - like connection.
A popular one is 'Fred, George and Hermione: A Magical Bond'. It's a fanfiction that focuses on the friendship between them. It shows how they support each other through different challenges at Hogwarts, like dealing with exams and the occasional dark magic threats. The way the author portrays their bond growing over time is really well - done.