The cervix sits at the bottom end of the uterus.
If you have felt inside your vagina you know that the hard, nubby bit is the Cervix.
During your menstrual cycle it’s position will change

During pregnancy, it stays firmly closed with a plug of mucous in the opening (the external os) to prevent any infection from travelling into the uterus. This plug will come out in the days, hours, minutes before the mother goes into labour and the cervix begins to soften.