A substance produced by the cervix and endocervical glands[BTO]. Thick acidic mucus that blocks the cervical os after mestruation[WP]. This 'infertile' mucus blocks spermatozoa from entering the uterus. [ BTO:0000242 http://en.wikipedia.org/wiki/Mucus#Reproductive_system http://en.wikipedia.org/wiki/Cervix#Cervical_mucus ]