foster mother vs surrogate mother

foster mother

noun
  • A female adult who cares for a child who has been placed in foster care. 

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