In this kind of fanfiction, their relationship could evolve in many ways. They would start to communicate on a deeper level, not just about their feelings for each other but also about their dreams for their family. They might also have to learn to forgive each other's mistakes quickly, because they are in a situation where they can't afford to hold grudges. Their love would be tested, but through it all, they could come out stronger and more in love, building a foundation for a family based on trust, respect, and mutual support.
Their relationship would likely become deeper as they face this huge life change together. They would have to rely on each other more, sharing fears and hopes. They'd be more united in their decisions regarding the pregnancy and their future.
Well, at first, they might have some arguments and misunderstandings because of the stress of the situation. But as time passes, they would realize that they need to support each other. Harry would become more protective of Ginny, and Ginny would be more understanding of Harry's concerns. Their relationship would grow from just a teenage romance to a more mature partnership as they prepare for parenthood.