foster mother vs mammy

foster mother

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

mammy

noun
  • In the southern United States, a black nanny employed to look after white children; or in the antebellum South, a female slave who was close to the household and looked after the children. 

  • mamma; mother 

How often have the words foster mother and mammy occurred in a corpus of books? (source: Google Ngram Viewer )