Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
nātīvĭtas, ātis, f. [nativus], birth, nativity (post-class.; cf. ortus): municipem aut nativitas aut adoptio facit, Dig. 50, 1, 1: janua nativitatis, Tert. Anim. 39: vultum nativitatis, natural face, Vulg. Jacob. 1, 23.