stepfather vs surrogate mother

stepfather

noun
  • A husband of one's biological mother after her initial marriage to or relationship with one's biological father. 

  • A husband of one's parent distinct from one's biological father 

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 stepfather and surrogate mother occurred in a corpus of books? (source: Google Ngram Viewer )