co-father vs paternity

co-father

noun
  • In a male homosexual couple, the nonbiological father (partner of the biological father) of a child. 

paternity

noun
  • legal acknowledgement of a man's fatherhood of a child 

  • authorship 

  • fatherhood, being a father 

  • parental descent from a father 

How often have the words co-father and paternity occurred in a corpus of books? (source: Google Ngram Viewer )