postmortem examination has 6 distinct synonyms spread across 1 of its 1 sense. 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.
an examination and dissection of a dead body to determine cause of death or the changes produced by disease
Synonyms, antonyms and the broader and narrower relations for postmortem examination come from Princeton University’s WordNet. Full sources and licences.