Ash would be very worried. He would immediately rush to Serena's side. He might stop whatever he was doing, like a Pokemon battle or exploring, and focus all his attention on her. He would probably ask if she's okay and check for any serious injuries.
Ash would be by Serena's side constantly while she recovers. He would tell her stories to keep her spirits up, like his adventures in other regions. He would also encourage her not to be too down about getting hurt and remind her of how strong she is. He might even postpone his own journey plans until he's sure Serena is fully healed.
In the fanfic, the reaction in the Pokemon world would be mixed. The local Pokemon townspeople might hold different views. Some would see it as a beautiful thing, a new start for Ash and Serena. Others might worry about how it will impact their Pokemon training. The Pokemon gym leaders could also have different reactions. Some might offer their gyms as a safe place for Serena to stay during her pregnancy, while others might be more hesitant, fearing it could set a precedent that might disrupt the normal flow of the Pokemon training world.
Brock would be more understanding. He's always been interested in girls, but he also values friendship. So he would probably offer his support to Ash, while also maybe giving some advice on how to act more 'ladylike' if that's what the situation calls for. As for the other Pokemon in Ash's team, they might not fully understand the concept of crossdressing, but they would sense that Ash is still their trainer and would protect him (or her in this case) as usual.
Pikachu, Ash's main Pokémon, would be very confused. It's used to Ash's mannerisms and commands. When Serena is in Ash's body, Pikachu might not respond as well at first. It might keep looking at 'Ash' quizzically, wondering why he's acting so strange. Serena's Pokémon, like Braixen, might also be confused. They might sense that something is off with 'Serena' (Ash in her body) and be a bit hesitant to follow commands as usual.
Serena could be a bit shy at first. She might blush deeply and look down for a moment. But then, she would look into Ash's eyes with love and admiration. She would tell Ash how much she loves him and how she has been hoping for this moment. She would accept his proposal gracefully, and they would share a tender embrace while their Pokémon friends cheer around them.
He would likely be heartbroken. Ash is a very emotional character. He would be confused and might start to question everything he knew about his mother. He could also try to resist in a non - violent way, hoping to bring his mother back to her normal self. He might call out to his Pokemon for help, not to attack his mother but to try and restrain her until he can figure out what's going on.
Pikachu would probably be very excited. It might jump around Ash, making little electric sparks of joy. Bulbasaur could bring Serena some fresh berries it finds in the forest as a sign of care. Eevee, if they have one, might curl up next to Serena's belly, as if protecting the unborn baby. Their Pokémon would sense the importance of this new development and do their best to be helpful and protective.
Some of their Pokémon friends might also show unique reactions. Pikachu could be very protective of Serena, always staying close by to make sure she's okay. And their other Pokémon might bring little gifts like berries or shiny stones they find, as if they understand what's going on. The more mischievous Pokémon might play little pranks that end up being endearing in the context of the pregnancy, like stealing Serena's hat and then replacing it with a flower crown as a sign of celebration.
Yuki would likely be very concerned. He has a soft spot for Tohru and would probably rush to her side, trying to find a way to help. He might even blame himself if he thinks he could have prevented it. Kyo, on the other hand, would initially be angry, but that's just his way of showing worry. He would be more hands - on in getting her the help she needs.
You can try some popular fanfiction websites such as FanFiction.net or Archive of Our Own. Just use the search bar and type in 'pokemon ash and serena hurt' to see what comes up.
One possible 'pokemon ash and serena hurt fanfiction' could be about Ash getting injured during a battle and Serena trying her best to take care of him. Maybe he's poisoned by a wild pokemon's attack and Serena has to find the antidote in a hurry.