The cervix is the lower part of the uterus (womb). It is sometimes called the uterine cervix. The body of the uterus (the upper part) is where a baby grows. The cervix connects the body of the uterus to the vagina (birth canal). The part of the cervix closest to the body of the uterus is called the endocervix. The part next to the vagina is the exocervix (or ectocervix). The 2 main types of cells covering the cervix are squamous cells (on the ectocervix) and glandular cells (on the endocervix). The place where these 2 cell types meet is called the transformation zone. Most cervical cancers start in the transformation zone.

Most cervical cancers begin in the cells lining the cervix. These cells do not suddenly change into cancer. Instead, the normal cells of the cervix first gradually develop pre-cancerous changes that turn into cancer. Doctors use several terms to describe these pre-cancerous changes,

including cervical intraepithelial neoplasia (CIN), squamous intraepithelial lesion (SIL), and dysplasia. These changes can be detected by the Pap test and treated to prevent the development of cancer (see "Can cervical cancer be prevented?").
Cervical cancers and cervical pre-cancers are classified by how they look under a microscope. There are 2 main types of cervical cancers: squamous cell carcinoma and adenocarcinoma. About 80% to 90% of cervical cancers are squamous cell carcinomas. These cancers are from the squamous cells that cover the surface of the exocervix. Under the microscope, this type of cancer is made up of cells that are like squamous cells. Squamous cell carcinomas most often begin where the exocervix joins the endocervix.

Although cervical cancers start from cells with pre-cancerous changes (pre-cancers), only some of the women with pre-cancers of the cervix will develop cancer. The change from cervical pre-cancer to cervical cancer usually takes several years -- but it can happen in less than a year. For most women, pre-cancerous cells will go away without any treatment. Still, in some women pre-cancers turn into true (invasive) cancers. Treating all pre-cancers can prevent almost all true cancers. Pre-cancerous changes and specific types of treatment for pre-cancers are discussed in the section, "Can cervical cancer be prevented?"

Pre-cancerous changes are separated into different categories based on how the cells of the cervix look under a microscope. These categories are discussed in the section, "Can cervical cancer be prevented?"