Well, there could be such a fanfiction out there. However, it really depends on the creativity and interests of the fanfiction writers in the relevant fandom. Some might find this concept interesting to write about, while others may not. It also might not be very common as it involves a rather specific and perhaps controversial plot point in the context of the characters.
Some alternatives could involve fanfictions centered around their individual character development. For example, a fanfiction about Natsu learning a new powerful spell or Mirajane exploring a new side of her magic. There could also be stories that place them in different settings, like a modern - day AU (alternate universe) where they are just regular people dealing with normal life problems while still maintaining their core character traits.
I'm not entirely sure if there is a specific 'natsu and mirajane pregnant fanfic'. Fanfic availability can vary greatly. Some fanfic authors might have explored this idea, while others may not. You could also try reaching out to fanfic communities on social media platforms and ask if anyone knows of such a fanfic.
Erza would probably be very protective. She would make sure Mirajane is taking proper care of herself and would offer practical advice on things like diet and exercise during pregnancy. Gray might be a bit awkward at first but would warm up and offer to help with any heavy lifting or tasks around their home.
There could be such a fanfiction. Fanfiction writers often come up with all kinds of creative and sometimes out - of - the - box ideas. However, I haven't personally come across one where Natsu gets Erza pregnant. It might be a rather niche concept within the Fairy Tail fanfiction community, as most fanfictions tend to focus on different aspects of the characters' relationships, like friendship, adventure - related partnerships, or other forms of romantic development.
I'm not sure if there is such a specific fanfiction. There are a vast number of fanfictions out there, and while some might explore unique and even out - of - the - ordinary concepts like this, it would take some searching through relevant fanfiction platforms to determine its existence.
It could be a story full of emotions. There would likely be a lot of anticipation and excitement. Natsu might be protective and a bit clumsy in his attempts to take care of Mirajane. And Mirajane would probably be strong yet vulnerable at the same time, dealing with the changes in her body and the new life growing inside her.
There might be a fanfiction where the pregnancy is a surprise to both. For example, in a story called 'Unexpected Miracles', it could start with Mirajane realizing she's pregnant and then Natsu's reaction being a mix of shock and joy. They then have to deal with the challenges of impending parenthood in a world full of magic and danger.
A plot could be that during a difficult mission, Mirajane discovers she's pregnant. Natsu then has to balance protecting her and completing the mission. This would add a layer of tension and emotion to the story.
In some fanfictions, it could be Gray. Their rivalry - turned - friendship has a lot of potential for such an unexpected plot twist where Gray is the father.