Marissa had a brief relationship with Ryan's friend, Luke Ward, before getting back together with Ryan. This resulted in Luke getting Marissa pregnant, but Ryan eventually found out he was the father after Marissa confessed she had a miscarriage.
This storyline, while not explicitly stating Ryan was the father, played with the ambiguity of who might be the father of Marissa's child. The reveal that Ryan was actually the father led to a significant emotional conflict in the series.