afterbirth vs amnion

afterbirth

noun
  • The placenta and other material expelled via the birth canal following childbirth or parturition in mammals. 

amnion

noun
  • The innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended. 

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