posthumous vs postmortal

posthumous

adj
  • After the death of someone. 

  • In reference to a work, published after the author's death. 

  • Taking place after one's own death. 

  • Born after the death of one's father. 

postmortal

adj
  • Synonym of post mortem 

How often have the words posthumous and postmortal occurred in a corpus of books? (source: Google Ngram Viewer )