Perhaps in the fanfic, they accidentally find out about the pregnancy while on an adventure. Ash would then become super protective of Serena. They could decide to take a break from their journey to find a safe place for Serena to have the baby. And during this time, they might have to fend off some villains who try to take advantage of their situation.
One possible storyline could be that Ash and Serena have been traveling together for a long time and their relationship deepens. They might face some initial shock and then start to plan for their future together. They could have to deal with reactions from their friends and family, like Pikachu being both excited and a bit confused.
Perhaps the story could involve a mystery related to Serena's pregnancy. There could be some strange events or prophecies in the Pokémon world that seem to be connected to the baby. Ash and Serena then have to go on a journey to figure out what it all means. Along the way, they meet new Pokémon and characters who either help or hinder their search for the truth.
One possible storyline could be about the challenges she faces during pregnancy, like dealing with morning sickness and hormonal changes. Another could be how her friends and family support her throughout the pregnancy journey. And perhaps there could be a storyline about her preparing for the new arrival, such as setting up the nursery.
One common storyline could be about their journey together as they prepare for the baby. It might include Ash being overly protective and Serena having to deal with her changing body and new emotions. Another could be a story set in a different Pokémon region where they face unique challenges as expectant parents, like dealing with new Pokémon species that are helpful or harmful during pregnancy.
I'm not sure about specific popular ones as this is a rather unusual and not very family - friendly concept in the context of the Pokémon series. Fanfictions often vary greatly in quality and content, and this particular theme might not be widely promoted in mainstream fanfiction communities.
Well, perhaps in the fanfic, the pregnancy could be a catalyst for Inuyasha and Kagome to bring their two worlds closer. They could start to work on making a place where their child can grow up safely, which means mending relations between demons and humans in their area. And of course, there would be the whole aspect of Kagome's pregnancy journey, like her changing emotions and physical needs.
Perhaps in the fanfic, Ron is away on some important mission when Ginny discovers she's pregnant. When he returns, he has to quickly adapt to the situation. They then decide to keep the pregnancy a secret for a while to avoid unwanted attention and drama within the wizarding community.
A common plot in Serena pregnant fanfic could be Serena's pregnancy being unexpected. She then has to rearrange her life plans. She might have to put her career on hold or find a new way to balance it with motherhood. There could also be a story where Serena is part of a community of pregnant women and they all share their experiences, which helps Serena grow and prepare for motherhood.
One idea could be that they reunite after a long time apart. Ash has become a more mature trainer and Serena has achieved her dreams too. They realize their feelings for each other have deepened. One passionate encounter leads to Serena getting pregnant. Then they have to deal with how their families and friends will react, and also how they will balance their future goals with raising a child.
One possible storyline could be that Ash is a knight in training and Serena is a noble lady. Ash has to prove his worth in a jousting tournament to win Serena's hand in marriage. Another could be that they are both on a quest to find a magical Pokemon that is said to have the power to save the medieval kingdom from an evil sorcerer. And a third could be that Serena is a healer and Ash is a warrior, and they team up to fight off a horde of dark - type Pokemon that are attacking the villages.
One possible storyline could be Serena feeling heartbroken and then deciding to focus on her own Pokemon journey independently. She might strive to become a better Pokemon trainer on her own terms, proving herself not just for Ash but for her own self - worth.