It could be her long - time boyfriend. They might have been planning for a family in the future, but the pregnancy comes as a bit of a surprise earlier than expected.
Maybe it's a mysterious stranger she had a brief encounter with. This would add a lot of mystery and drama to the story as she tries to figure out who he is and how to handle the situation.
In some fanfiction, it could be an enemy or rival who, through some unexpected turn of events, is the father. This would create a very complex and tense relationship as they have to deal with their existing animosity while also being connected through the baby.
It could be an old flame of Lydia's. Maybe someone she had a passionate relationship with in the past and they had a brief reunion which led to the pregnancy.
Stiles would be a very likely candidate as they have a close relationship in the show. Their banter and connection could easily lead to a situation where they end up having a baby together in a fanfiction.
A new, unknown supernatural entity that feeds on the energy of unborn children with special powers. It senses the unique nature of Lydia and Parrish's baby and starts to haunt them. It causes all kinds of problems like making Lydia sick, or creating nightmares for Parrish, all in an attempt to get closer to the baby so it can feed on its energy.
It could be one of the main male characters in One Tree Hill. Maybe Nathan Scott, which would create a really complex family dynamic since he's already part of the Scott family.
It could also be someone new, a character created specifically for the fanfiction. Maybe a powerful new demigod who catches Annabeth's eye, and their relationship leads to the pregnancy.
It could be a long - lost love from Mavis's past. Maybe someone she thought she'd never see again but who re - enters her life right before she discovers she's pregnant.
It could be another powerful magical being in her realm. Maybe a long - time ally or a mysterious figure that has been in the background of the story's world. This would add an element of mystery and intrigue to the fanfiction.
In some fanfictions, the father could be a human character. This would add an interesting dynamic as it would involve dealing with the differences between a human and a Digimon in a relationship and during pregnancy. It could also lead to a story about acceptance and dealing with the unconventional nature of such a union.
It could be one of her close friends or classmates in 90210. Maybe someone she had a brief fling with during a party or a school event.
Maybe it's a new character introduced just for this plot twist. This would add an element of mystery and new drama to the story.