co-father vs surrogate mother

co-father

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

surrogate mother

noun
  • A woman who gives birth to a child not her own, with the intent to give that child up to at least one of the biological parents once it is born. 

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