The poion of the cervix exterior to the external os is called ectocervix. It is lined predominantly by nonkeratinized stratified squamous epithelium. The endocervical canal is covered by a single layer of mucin-secreting columnar epithelium. During pregnancy mucus produced by the endocervical epithelium becomes thick and forms a mucus plug within the endocervical canal. During pregnancy, the endocervical epithelium moves out and onto the ectocervix during enlargement of the cervix. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 2. Maternal Anatomy.