In this fanfiction, they could face the challenge of dealing with the baby's health issues. Even a small cold can be very worrying for new parents. They may not know when to take the baby to the doctor or how to soothe the baby when it's sick. Additionally, they might face social isolation. Their friends may not understand what they're going through, and they might find it hard to maintain their previous social life with a new baby in the picture.
One challenge could be lack of sleep. Newborns need to be fed and cared for at all hours, so Marcus and Bella might be constantly tired. Another might be financial stress. Babies need a lot of stuff like diapers, clothes, and formula. They could also face family interference. Maybe their families have different ideas on how to raise the baby, which could cause arguments between Marcus and Bella.
Well, in the fanfiction they might face the challenge of adjusting to their new roles. Marcus may struggle with not having as much freedom as before, and Bella might find it hard to balance taking care of the baby and her own needs. Also, if they have different parenting styles, for example, Marcus is more laid - back and Bella is more strict, it could lead to conflicts when making decisions for the baby.