biological mother vs mammy

biological mother

noun
  • The woman from whom one inherits half of one's DNA and who is the source of one's mitochondrial DNA. 

mammy

noun
  • mamma; mother 

  • 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. 

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