postnatus

from The Century Dictionary.

• Referring to a second son; referring to a person born after a particular event. See the noun.
• noun In law: The second son.
• noun One born after a particular event: as, one born in the United States after the Declaration of Independence (1776) is a postnatus; a postnatus in Scotland is one born in that country after the accession (1603) of James VI. to the English throne as James I. Compare antenati.