The cervix (or neck of the uterus) is the lower, narrow part of the uterus which joins to the top end of the vagina. The opening of the cervix is called the os. The cervical os allows menstrual blood to flow out from the vagina during menstruation. During pregnancy, the cervical os closes to help keep the foetus in the uterus until birth. During labour, the cervix dilates, or widens, to allow the passage of the baby from the uterus to the vagina. Approximately half the cervix length is visible with appropriate medical equipment; the remainder lies above the vagina beyond view.
The cervix is covered with a layer of skin-like cells on its outer surface, called the 'ectocervix'. There are also glandular cells lining the inside of the cervix called the endocervix. These cells produce mucus. The skin-like cells of the ectocervix can become cancerous, leading to a squamous cell cervical cancer. Or the glandular cells of the endocervix can become cancerous, leading to an adenocarcinoma of the cervix.
The ectocervix and endocervix have a three main skin layers or zones:
- The basal layer - cells are produced here. Older cells are pushed up towards the surface. If you contract HPV, the virus will attack the basal layer cell.
- Midzone – the middle layer of cells. As cells move up from the basal layer they lose their capacity to divide making them fully mature cell.
- Superficial zone – The uppermost surface of the cervix where mature cells eventually die and shed in the normal process of skin shedding  . Cervical screening takes cells from this area.
The area where cervical cells are most likely to become cancerous is called the transformation zone. This is the area just around the opening of the cervix that leads on to the endocervical canal (the narrow passageway that runs up from the cervix into the womb). The transformation zone is the area that your doctor or nurse will concentrate on during cervical screening.
The vagina is the tube from the outside of the body to the entrance to the womb. The skin-like cells that cover the cervix join with the skin covering the inside of the vagina, so even if you have had your womb and cervix removed, you can still have screening samples taken from the top of the vagina.