One way this could happen in fanfiction is if there was some form of time - travel involved. Maybe Hermione traveled back in time and ended up in a situation where she and Severus had a relationship that led to her pregnancy. Another possibility is that a magical artifact forced them into a situation where they had to be together closely, and feelings developed over time, resulting in a pregnancy. Fanfiction writers love to take unexpected turns with characters' relationships.
In the context of fanfiction, this could happen due to Severus' long - standing love for Lily. Maybe in this particular fan - created story, Severus realizes that his true feelings for Lily were never fully extinguished. Hermione, on the other hand, might not have been able to fill the void that Lily left in Severus' heart. It could also be that the author wants to explore a more complex and perhaps angsty relationship dynamic where Severus is pulled back to his first love, Lily, despite having some sort of relationship with Hermione.
It could be that in the fanfiction world, Hermione was the victim of a spell gone wrong. This spell could have made her pregnant and also somehow cut her off from her usual support system, leaving her alone. Or perhaps she made a choice to distance herself from others because she didn't want to endanger them while she was dealing with this unexpected pregnancy.
One way it could happen is if Hermione discovers a time - turning device in the Marauders' era. She might use her advanced knowledge to figure out that Snape is in danger. For example, she could know about some plot by the Marauders to prank Snape in a way that could turn deadly. She then steps in, using her magic skills and intelligence to prevent the harm. Maybe she creates a diversion or uses a protection spell to shield Snape.
There could be a plot where they are forced to work together on a project. Snape initially resents it, but as they collaborate, he sees Hermione's determination and begins to fall for her. However, he has to deal with the fact that she is a Gryffindor and he has his own reputation to uphold. This creates a lot of drama and tension in the story.
Harry would probably be extremely shocked. He has a deep respect for both Hermione and Severus in different ways, and this situation would completely turn his world upside down. He might initially be angry or confused, but then try to understand what led to this. Ron, on the other hand, would likely be more openly outraged. He has always had a bit of a jealous streak when it comes to Hermione, and this would be a very difficult situation for him to accept.
In this fanfiction, the pregnancy could be a result of a magical accident. Hermione and Blaise are working on a powerful spell, and something goes wrong. Now they have to deal with the consequences. Hermione might be scared but also determined to face it. Blaise would need to step up and support her. They might have to go on a journey to find a way to reverse the spell if they don't want the pregnancy, or they could accept it and start building a family. Along the way, they would face many obstacles, like other wizards who might want to use the situation for their own gain.
Fanfiction often takes beloved characters and places them in unexpected situations. In the case of Hermione and Severus, a pregnancy fanfiction could explore a very different side of their relationship. It might be set in an alternate universe where they have a closer bond than in the original story. However, it's important to note that this goes against the canon relationship between them in the Harry Potter series.
Well, they could start by trying to figure out a living arrangement. Since they come from different backgrounds, it might be a bit of a struggle. Hermione might want to bring in some Muggle - like elements into the child's upbringing, while Draco is more used to the pure - blood ways. But they could compromise. For instance, they could decorate the child's room with both magical and Muggle items.
Perhaps Hermione discovers a new form of magic during her pregnancy. Since she is so brilliant, this new magic could be related to protecting the unborn child. And Antonin could be the one who encourages her to explore this new magic, which in turn deepens their relationship.
Magic could play a role. There could be a magical spell or potion that was involved in Hermione's conception that links her to Snape. For example, a potion that alters memories and relationships. This would make for a very interesting and complex backstory in the fanfiction. Snape might not even be aware that he is Hermione's father at first, and when he discovers it, his whole view of her would change. He would have to deal with his feelings of protectiveness and his past actions towards her in Hogwarts.