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.
Most of their Pokémon would show great love and care for the baby. Ash's Charizard, despite its large and sometimes intimidating size, would be very gentle. It could use its warm body to keep the baby cozy during cold nights. Serena's Pokémon would also contribute to the baby's upbringing. For instance, her Pancham could be like a little playmate for the baby as it grows up. The Pokémon would also be a big part of the baby's education about the Pokémon world, teaching it about different types and how to interact with them in a positive way.
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.
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.
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.
Their Pokémon might be very protective. For example, Pikachu could stay closer to Serena all the time, sensing the new life. Some Pokémon might bring small items like berries or soft leaves, thinking it could be useful for Serena or the baby.
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.
There are many ways fanfiction writers can take this concept. They could write about the baby having special powers or a unique connection to Pokemon. It could also be a story about the family traveling together, with Ash and Serena passing on their knowledge and values to their child. This type of fanfiction can be a great exploration of family dynamics within the Pokemon universe.
At first, his Pokémon would be startled. They've always known Ash as a human. But then, they would start to adapt. Some Pokémon, like Squirtle, might try to play with Ash in his new form, thinking it's a new kind of game. However, his more loyal Pokémon, like Greninja, would be determined to find a way to turn Ash back. Greninja might use its special bond with Ash to try and communicate with him on a deeper level in this new form, to understand what happened and how to reverse it.
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.
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.
His Pikachu would be heartbroken. Pikachu has been with Ash from the very beginning, and it would likely go into a state of deep depression. It might not eat or interact with others for a long time. Other Pokémon like Charizard would also be extremely sad. Charizard, which has a strong bond with Ash, might fly off on its own, looking for a place to grieve in solitude.