amniotic sac vs hymen

amniotic sac

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

hymen

noun
  • A membrane which completely or partially occludes the vaginal opening in human females. 

  • Marriage. 

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