word-forming element meaning "after," from Latin post "behind, after, afterward," from *pos-ti (source also of Arcadian pos, Doric poti "toward, to, near, close by;" Old Church Slavonic po "behind, after," pozdu "late;" Lithuanian pas "at, by"), from PIE *apo- (source also of Greek apo "from," Latin ab "away from" see apo-).
late 14c., "of or pertaining to birthdays;" mid-15c., "of or pertaining to one's birth," from Latin natalis "pertaining to birth or origin," from natus, past participle of nasci "to be born" (Old Latin gnasci), from PIE root *gene- "give birth, beget." It is the learned form of Noel, which was the French vernacular word.
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<a href="https://www.etymonline.com/word/postnatal">Etymology of postnatal by etymonline</a>
Harper, D. (n.d.). Etymology of postnatal. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/postnatal