In Desiree's Baby, Kate Chopin focuses in on a tragic set of circumstances which result in a surprise ending.
The theme of background is illustrated through the fact that no one knows Desiree's ancestry because she was found by the side of the road. Her mother tells her intended, Armand, that they have no idea where she came from, but they adopted her without question.
Armand marries Desiree claiming that he doesn't care that she has no name, he will give her a new name, his. The trouble begins to become apparent when they have a child, a son, and the boy exhibits traces of African ancestry. Desiree, who does not know her biological parents, realizes that Armand has rejected her because of the child's appearance.
Desiree's identity appears to be clarified by the child's appearance, so she experiences a sudden loss of personal worth, which leads her to take the child into the Bayou and never be seen again.
The rejection she feels, she believes is because she has been exposed as being part African. However, as Armand clears away every remnant of Desiree's existence, he discovers a letter from his mother, which reveals a shocking secret. He is the one who has African ancestry in his background. The African features of the child came from his side of the family, not from Desiree.
Armand's promise to love Desiree no matter what her background, was a lie. He rejects her based on mistaken identity.