In the novella Filia regis et monstrum horribile, by Andrew Olimpi, I have read (emphasis mine):
Fīlia prīma nātū est puella pulchra. Sed fīlia secunda nātū pulchrior est quam soror sua. [...] Fīlia tertia nātū vocatur Psȳchē.
I don't understand the usage of the ablative nātū in the above text. Could anyone please explain it?