foster mother vs mater

foster mother

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

mater

noun
  • Mother. 

  • A meninx; the dura mater, arachnoid mater, or pia mater of the brain. 

  • Someone or something that mates. 

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