There is no such fanfiction that is appropriate or ethical. Pokémon are fictional creatures and the idea of a human and a Pokémon in such a situation violates the nature and ethics of the Pokémon universe. So, you won't find it in any legitimate fanfiction platforms.
Most likely, this is a very niche and perhaps even inappropriate concept for fanfiction. The Pokémon world and the human world are kept quite distinct in the official lore, and crossing those boundaries in such a way as a 'pregnant' scenario goes against the general spirit of the Pokémon franchise. So, it's doubtful there are many, if any, proper fanfictions on this.
Well, one possible interpretation could be a very strange and unique take on the relationship between Ash and Pikachu. Maybe it's a metaphorical exploration of their deep bond in a really extreme way. But this is a very odd concept.
There is no such official or acceptable fanfiction. Pikachu is a Pokémon and Ash is a human character, and such a concept goes against the nature of the Pokémon world and is also inappropriate and unethical.
I'm not sure if there is specifically a 'Ash gets Iris pregnant' fanfiction. There are countless fanfictions out there, but this particular theme might be quite rare or even non - existent depending on the nature of the Pokémon fan community's standards and the general focus of fan - created stories. Fanfictions usually range from adventure - based to friendship - focused within the Pokémon universe.
In the 'Ash gets a female Pikachu as a starter fanfiction', one possibility is that the female Pikachu could form special bonds with other Pokémon. Since female Pokémon often have different social behaviors, she could befriend different types of Pokémon more easily. This would open up new storylines for Ash, like being able to access areas or get help from Pokémon that he otherwise wouldn't have. Also, she might be more attracted to certain types of Pokéballs or items, which could also add interesting twists to the story.
It could be Team Rocket as always. Since Ash is now a Pikachu, they might see it as an easy target to capture. They could have new gadgets specifically designed to catch Pikachu - Ash, and constantly try to get their hands on him throughout the fanfiction.
One possible plot could be that Ash is turned into a Pikachu by a strange spell cast by a mischievous Pokémon. As a Pikachu, he has to figure out how to communicate with his friends. His Pokémon pals at first don't recognize him, but Pikachu - Ash starts using his old battle strategies in a new way. For example, he tries to direct Pikachu's electric attacks in the same patterns he used to direct his other Pokémon's moves in battles. Eventually, his friends realize it's him and they set out to find a way to reverse the spell.
Well, he might start to see the world from a Pikachu's perspective. For example, he would experience the small size and how everything looks so huge. He could also find it difficult to communicate with his human friends in the normal way since he is now a Pikachu. Maybe he would have to rely on Pikachu's ways of expressing emotions like using electrical signals.