The Role of the Cervical Os in Pregnancy and Childbirth

Anatomical model of baby in a woman's womb

Dan Prince / Getty Images

In This Article
Table of Contents

The cervix is located in the pelvic region within the female reproductive system. It is typically just over 1 1/2 inches in length and comprises a matrix of collagen-cell tissue lined with a single layer of cells bridging the distance between the vagina and the uterus. There are four main areas of the cervix:

  • Ectocervix: Non-mucus producing cells that form the lower third of the cervical canal
  • Endocervix: Mucus-producing cells that form the upper two-thirds of the cervix
  • External os: The opening on the outermost, vaginal side of the canal
  • Internal os: The opening on the innermost side of the canal near the baby

The Role of Cervical Os in Menstruation

Throughout a woman’s menstrual cycle, the internal and external cervical os will open and close in conjunction with the various phases of menstruation. Depending on the stage, the position of the cervix will change, sometimes moving higher and sometimes moving lower.


During ovulation, the cervix will be high and positioned nearer to the top of the vagina. This change of position will allow the cervical os to open more readily to allow sperm to enter.

The secretion of cervical mucus will further accommodate the sperm by altering the environment of the vagina from its natural acidic state to a more alkaline one. To better ensure the sperm can make their way to the cervical os, the mucus will also become thinner and clearer.

Non-Fertile Stage

During the non-fertile stage of menstruation, the cervical position will be lower and the cervical os will close. Vaginal secretions will become thicker and more acidic to protect against bacteria and other infective agents.

The Role of Cervical Os in Pregnancy

Just as it changes shape and position before a woman is pregnant, after conception and the implantation of the fertilized egg in the uterus, the cervical os changes in response to the stage of pregnancy and growth of the developing fetus.

As the pregnancy progresses from the second to third trimester, the fetus will begin to descend into the uterus in preparation for birth. The role of the cervix at this stage is to provide stable support for the baby’s head as it pointed head-first toward the cervical canal.

Labor and Childbirth

As labor progresses, the cervix will become softer and shorter, and the cervical os will unilaterally begin to dilate. To accommodate the head of the baby during childbirth, the entire cervix canal must open to a diameter of more than 4 inches (10 centimeters). In becoming wider, the cervix will also become shorter and thinner, a phenomenon known as effacement.

In the process of effacement, the internal and external os will come closer together. As effacement and dilation progress, the doctor or midwife will use the extent of the cervical opening to help make treatment decisions.

  • During the first stage of labor, the uterine contractions will become strong and regular and the cervical will dilate to more than 2 inches (5 centimeters).
  • During the second stage, the cervix will open to the full dilation of 4 inches (10 centimeters); this is when active pushing begins to deliver the baby through the birth canal.
  • Immediately following childbirth, the internal cervical os closes while the external cervical os may remain dilated for several weeks.

Self-Examination of the Cervical Os

You can locate the cervical os during a simple self-exam. By inserting two clean fingers into your vagina, you can feel the cervix to the back of the vaginal canal. It may help to gently press on the lower abdomen with your other hand to push the cervix closer to the vaginal canal.

The cervix will feel round, while the cervical os will feel like a small donut with a hole or indentation in the middle.

If planning to conceive, you can use the quality and position of the cervix and cervical os to help you determine whether you are entering a fertile phase:

  • When ovulating, the cervical os will be open and relaxed, and the surrounding cervix will be high and soft and feel similar to pursed lips.
  • During a non-fertile phase, the cervix will be low and firm, and the cervical os will be closed.
Was this page helpful?
Article Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. SEER Training Modules. Anatomy of the Female Pelvis. National Cancer Instittute (NCI). Published online, no date.

  2. Women's Health Queensland Wide. Understanding Your Menstrual Cycle Fact Sheet. Published online, updated July 2015.

  3. Myers KM, Feltovich H, Mazza E, et al. The mechanical role of the cervix in pregnancy. J Biomech. 2015;48(9):1511-23. doi:10.1016/j.jbiomech.2015.02.065

  4. Vink J, Myers K. Cervical alterations in pregnancy. Best Pract Res Clin Obstet Gynaecol. 2018;52:88-102. doi:10.1016/j.bpobgyn.2018.03.007

  5. Bialy A, Wray AA. Gynecologic Examination. StatPearls online, updated April 8, 2019.

Additional Reading
  • Simkin, P. and Ancheta, R. (2011) The Labor Progress Handbook (Third Edition). New York: Wiley-Blackwell.