What is unique about Desiree's birth?

Desiree's birth is unique because it is a complete mystery. No one knows her heritage, she was simply found.

Why, it seemed but yesterday that Désirée was little more than a baby herself; when Monsieur in riding through the gateway of Valmondé had found her lying asleep in the shadow of the big stone pillar.

The prevailing belief was that she had been purposely left by a party of Texans, whose canvas-covered wagon, late in the day, had crossed the ferry that Coton Maïs kept, just below the plantation. In time Madame Valmondé abandoned every speculation but the one that Désirée had been sent to her by a beneficent Providence to be the child of her affection, seeing that she was without child of the flesh. For the girl grew to be beautiful and gentle, affectionate and sincere,—the idol of Valmondé.