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.
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.
Maybe in this fanfiction, Ash has been planning the proposal for a long time. He enlists the help of all their Pokémon friends. Pikachu holds the ring in its tiny paws. Ash takes Serena to a place that holds special memories for them, like the place where they first met. He then gives a heartfelt speech about their journey together and how he wants to continue it as her partner forever before popping the question.
They would probably be a bit shy at first. Ash being his usual energetic self but also respectful of Serena's space in the tent. Serena might be a bit nervous but also excited to be sharing such a close space with Ash.
Their friends would be overjoyed. Brock would probably offer his medical knowledge to make sure the baby is healthy. Misty might be a bit jealous at first but would soon warm up and offer to teach the baby how to swim. Clemont could create some amazing gadgets for the baby, like a Pokémon - themed mobile or a stroller that can move on different terrains easily.
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.
Their Pokémon would probably be very protective. Pikachu, being Ash's closest companion, would always be on guard around the baby. It might even try to play with the baby in its own cute way, like using its electric shocks to create little light shows to amuse the baby. Serena's Pokémon, like Fennekin which evolved into Delphox, could use its psychic powers to sense if the baby is in any danger or discomfort. All of their Pokémon would likely adapt their usual habitats and behaviors to be more baby - friendly, for example, making sure not to use any dangerous moves near the baby.
Obviously, the main characters are Ash and Serena. Ash is the protagonist who has been on many Pokemon adventures, and Serena is his friend and love interest in this fanfiction.
Well, it could be that Ash finally realizes his deep feelings for Serena during a grand Pokemon event. Maybe they are in a beautiful, flower - filled meadow near a Pokemon Center, and Ash gets down on one knee with a Pokemon - themed ring, perhaps made from a shiny Pokemon scale or something unique.
Their relationship would become more mature. They would have to rely on each other more than ever. With a baby, they'd have to share responsibilities, which could lead to a deeper understanding and respect for each other.