mom vs surrogate mother

mom

noun
  • Mother. 

  • An adult female owner of a pet. 

verb
  • To care for in a motherly way. 

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