The short story "Desiree's Baby" by Kate Chopin takes place in the deep south in the state of Louisiana. The setting of the story is before the Civil War. None of this information is stated in the story but from clues the reader can draw these conclusions.
There are still slaves on Armand's plantation [prior to the Civil War when slaves were freed]. The Valmondes speak French which was the second language in Louisiana. From these inferences, the reader knows now that being a mulatto which would have been half black and half white would have been unacceptable in the society of both the Valmondes and Aubignys. The upper and middle classes in Louisiana would never have accepted a bi-racial child as their equal.
When it was discovered that Desiree's child had negroid features, the obvious person to have the black heritage was Desiree. Her ancestry was unknown. Therefore, everyone believed-- including her parents, Desiree herself, and Armand--that it had to be Desiree.
What no one knew except for Armand's parents was that he too was adopted. However, they did know his heritage. From the letter read by Armand at the end of the story, he was bi-racial. The letter stated:
'But above all,' she wrote, 'night and day, I thank the good God for having so arrange our lives that our dear Armand will never know that his mother, who adores, him belongs to the race that is cursed with the brand of slavery.'
His mother was black. The child's negroid features missed Armand, and in the second generation, are found in his child.