Actually, in the actual novel,the miracle baby was one of the plot though. When Alessia discovered that she's pregnant, it was when she and Sebastian still didn't make up. So Alessia suggested that they got a divorce (since Sebastian supposedly couldn't stand her) because the condition is fulfilled (because she had produced an heir). But Sebastian of course will not have any of that. I think that was the final turning point of their relationship.

Did they have to include a miracle-baby? Couldn't they just have adopted?