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.
In the fanfiction, it could be that the pregnancy is a turning point in their lives. Lydia and Aiden could be part of a fantastical world where the pregnancy has some special meaning or power. For example, the baby could be prophesied to bring great change to their world. So they not only have to deal with the normal aspects of pregnancy like doctor visits and baby showers, but also protect the unborn child from those who might want to use its power for evil.
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.
It could be Ron. Since they have a relationship in the original story, it's a very likely option. Their relationship could be further developed as they face the new chapter of becoming parents together.
Perhaps it's a human boy who accidentally wandered into the fairy world. Tinkerbell, being curious, interacted with him, and in a magical turn of events, she became pregnant. This would create a lot of drama as the fairy world and the human world don't usually mix in such an intimate way, and there would be a struggle to figure out how to handle this situation.
Another possibility is that it's a result of some sort of experimental jutsu gone wrong. So there may not really be a traditional 'father' in the sense, but rather it's a consequence of Naruto's own actions in the world of ninja magic.
It could be her long - time boyfriend. They've been in a loving relationship, and this pregnancy is a natural step in their relationship journey.