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.
Another plot could involve some sort of magical or supernatural element related to the pregnancy. Since Sabrina has a connection to magic, perhaps there are some strange occurrences during the pregnancy. It could be that the unborn baby has some sort of special power that starts to manifest early, and they have to figure out how to protect the baby and themselves from those who might want to use the baby's power for their own gain.
A third plot idea is that they initially struggle with the idea of becoming parents so young. They might consider different options like adoption at first. But as the pregnancy progresses, they start to bond with the baby and gradually come to terms with the idea of raising a child. They could also have a support system in the form of their closest friends who help them through the difficult times and encourage them to embrace this new chapter in their lives.