In this kind of fanfic, the plot could be quite complex. First, they need to figure out how they got pregnant, whether it was planned or not. Then, they face challenges like financial concerns if they are young. Their relationship might also be tested. Sabrina may have to hide her pregnancy from some in the magical world who might not approve. Harvey could be thinking about his future and how to support his family. There could also be a sub - plot about their friends helping them or some unexpected magical interference in the pregnancy that they have to deal with.
In the most common comic interpretations, Harvey and Sabrina do end up together. However, there are alternate storylines where this might not be the case. It all comes down to the creative choices of the comic writers.
In some cases, it could be a powerful magical being from Sabrina's world. This being might think that the pregnancy is a bad omen or against some magical law. So, they try to stop the pregnancy or make Sabrina and Harvey's lives difficult by creating all sorts of magical obstacles for them.
One idea could be that they discover the pregnancy unexpectedly and have to deal with how it will affect their future plans. For example, their education or career aspirations. Maybe Sabrina is worried about how her family will react, while Harvey is trying to be the supportive partner. They could face challenges like financial concerns or social pressure from their friends and community.