Definition of Cervical Os

A Part of the Female Reproductive System

The cervix as it is seen pre vaginal delivery (left), with a rounded os, and post vaginal delivery (right), with a linear or horizontal os. CFCF/Wikimedia Commons

Definition: The cervical os is part of the female reproductive system and is located in the pelvis. It is part of the cervix,which is in the lower part of the uterus. The cervix is about two inches in length but can vary in length and width during a woman's lifetime. There are two main portions of the cervix:

The ectocervix, the passage between the uterus and vagina. The external os is the opening in the center of the ectocervix.

The endocervix, also known as the endocervical canal, is a passage that connects the vagina to the uterus. The internal os is the opening into the uterus from the cervix. 

The border that overlaps between the ectocervix and the endocervix is known as the transformation zone. 

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