amma vs mother-in-law

amma

noun

  • mother 

  • An abbess or spiritual mother, especially one of the Desert Mothers. 

mother-in-law

noun

  • The mother of one's spouse. 

  • A mother-in-law sandwich. 

  • A mother-in-law apartment. 

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