fiancée vs sweetheart

fiancée

noun
  • A woman who is engaged to be married; the woman to whom one is engaged. 

sweetheart

noun
  • A person very much liked or loved by someone, especially when both partners are young. 

  • A person who is always very kind. 

  • A female member of a college or university fraternity. 

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