It could be that the pregnancy is a sign of a prophecy coming true in a very unexpected way. In the wizarding world, prophecies often have strange and convoluted ways of manifesting. Harry might discover that this pregnancy is linked to a greater destiny for him and the wizarding world. Maybe the child he's carrying will be a powerful force for good or evil, depending on how things play out. His relationship with Ginny would also be greatly affected. She would have to deal with her own feelings about Harry's situation, and they would need to work together to face this extraordinary event.