afterbirth vs amniotic sac

afterbirth

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

amniotic sac

noun
  • The sac in which the foetus develops in mammals, reptiles and birds. 

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