spontaneous abortion vs stillbirth

spontaneous abortion

noun

  • A miscarriage. 

stillbirth

noun

  • The birth of a dead fetus after 20 weeks of gestation. 

  • The birth of a dead fetus; the delivery of an infant which is dead at birth. 

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