co-father vs single parent

co-father

noun

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

single parent

noun

  • A parent who is the only guardian of offspring. 

verb

  • To act as the only guardian of offspring. 

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