Often, the pregnancy is depicted in a way that ties into the overall themes of the Supernatural world. It could be part of a prophecy or some sort of dark magic. Maybe she was cursed with the pregnancy, and it's now a race against time to either get rid of it or use it to her advantage. Fanfic writers might show her dealing with the physical and emotional toll of pregnancy while still trying to maintain her status among demons.
One common theme is power struggle. Abaddon is a powerful demon, so many fanfictions explore her battles for power, either against other demons or the Winchesters. Another theme is her backstory. Writers often delve into her past to give more depth to her character.
I'm not sure specifically which 'Henry Abaddon's Daughter' fanfiction in Supernatural you're referring to. There could be many different takes on this in the fanfiction world. Some might focus on her relationship with the main characters in the show, like how she might interact with Sam and Dean. Others could explore her backstory, her powers if she has any, and how she fits into the Supernatural universe.
One common theme is the struggle between good and evil. Since Abaddon is a demon, her daughter often has to deal with the pull of her demonic heritage while also having the potential for good. For example, in many fanfics, the daughter may be tempted to follow in her mother's footsteps but then has to fight against that urge.
One key element is the power dynamic. Henry, being a Winchester, has his own set of skills and knowledge, while Abaddon is a powerful demon. Their interactions often revolve around who has the upper hand at any given moment.
One common plotline is the mystery behind the pregnancy. Is it a result of a forbidden love with another demon or some powerful entity? Writers often use this to create suspense.
There's also a fanfic titled 'Abaddon's Daughter: Redemption'. In this one, the daughter of Abaddon tries to break free from her mother's dark legacy. It has some really interesting plot twists and the relationship between the main character and the other Supernatural characters like Sam and Dean is well - developed. It's a great read if you're into character - driven stories.
The 'oc x abaddon supernatural fanfiction' is appealing because it allows fans to explore uncharted areas of the Supernatural universe. The OC can introduce new storylines and concepts that might not be possible with the existing characters. Also, the relationship between the OC and Abaddon can range from romantic to adversarial, and seeing how this unfolds in different fanfictions is part of the allure.
You can start by looking on fanfiction.net. It has a vast collection of Supernatural fanfictions, including those about Abaddon. Another great place is Archive of Our Own (AO3). There are many talented writers on these platforms who create interesting stories centered around Abaddon.
There's also 'Henry and Abaddon: An Unlikely Alliance'. In this fanfic, it shows how Henry and Abaddon might have to team up against a common enemy in the Supernatural universe. It's full of action and some great character development for both of them.