There's no way Nathan's birthday could be in December for many reasons. He's supposed to be younger than Lucas, but if their in the same grade and Lucas's birthday is in May and Nathan in December that would make him older. It was also stated in the show that Lucas is three months older than Nathan. Well, with the birthdays mentioned it would make Nathan 5 months older than Lucas. The show also told us Karen got pregnant right out of high school and since Lucas's birthday is listed as May 20th that would mean conception occurred sometime in August, just after high school. However, if Nathan is three months younger than Lucas it would mean he was conceived three months later in November making his birthday in August, three months after Lucas.