In a 'Marrying Father Christmas Pregnant' fanfiction, the plot could be quite complex. Maybe there's a prophecy that says Father Christmas must marry a mortal to save Christmas. So he marries this woman, and she soon finds out she's pregnant. The pregnancy could be a symbol of new hope for Christmas. As she's pregnant, they have to face the jealousy of other magical beings who thought they had a chance with Father Christmas. And the couple has to find a way to combine their two very different worlds to raise the child in a way that benefits both the magic of Christmas and the normal life of the mother.
Well, the plot might start with a young woman who has a special connection to the Christmas spirit. She meets Father Christmas during one of his deliveries and they fall in love. After getting married, she becomes pregnant. The story could then focus on the conflict between the normal world she came from and the magical world of Father Christmas. Her family might not understand, and there could be issues with how the baby will grow up, whether it will have normal human qualities or special Christmas - related powers.