birthright has 1 distinct synonym spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a right or privilege that you are entitled to at birth
“free public education is the birthright of every American child”
an inheritance coming by right of birth (especially by primogeniture)
Synonyms, antonyms and the broader and narrower relations for birthright come from Princeton University’s WordNet. Full sources and licences.
personal characteristics that are inherited at birth