(Being rewritten) Nightmare Moon was pregnant with Twilight Sparkle when she was banished. How does this change events?

After her coronation, Twilight receives a letter from Celestia telling her she has officially graduated. When she goes to confront her, she finds Celestia having tea with a younger unicorn student.

Little filly Twilight seeks comfort in her brand new mentor during a scary holiday

Before the banishment of Luna, was a tragic story of love and war. Join me, young ponies, as I tell you of the Student of the night and the war that follows.