fetus vs unborn child

fetus

noun
  • A human embryo after the eighth week of gestation. 

  • An unborn or unhatched vertebrate showing signs of the mature animal. 

unborn child

noun
  • A human offspring that is developing in the uterus; fetus. 

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