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.
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.
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.
You can start by looking on fanfiction websites like Fanfiction.net or Archive of Our Own (AO3). Just search for 'baby Fred and George' in the search bar and you'll get a list of relevant fanfictions.
One great baby Fred and George fanfiction is 'The Twins' Early Adventures'. It shows their mischievous nature even as babies in a really cute way. Another one could be 'Baby Fred and George's Magic Pranks' which focuses on the little magical things they do that foreshadow their future as pranksters at Hogwarts.
Yes, there might be fanfictions out there with this concept. Fanfiction writers often come up with creative and unexpected pairings. However, this is a rather unusual pairing considering Hermione is typically associated with Ron in the original 'Harry Potter' series. You can try searching on major fanfiction websites like Fanfiction.net or Archive of Our Own. Just type in relevant keywords to find stories based on this idea.
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.
One possible plot could be that Hermione and Fred start off as friends who gradually develop deeper feelings. Maybe they work together on a project in the wizarding world that brings them closer. Then, an unexpected moment of passion leads to Hermione getting pregnant. The story could then focus on how they deal with the news, including telling their families and friends in the wizarding community, and how they plan for their future together.