The Role of the Cervical Os in Pregnancy and Childbirth

Anatomical model of baby in a woman's womb

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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 bridges the distance between the vagina and the uterus. The cervix is comprised of a matrix of collagen-cell tissue lined with a single layer of cells. 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 uterus

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 in the vagina.

Ovulation

During ovulation, the cervix is high and positioned nearer to the top of the vagina. This allows the cervical os to open more readily to allow sperm to enter.

The secretion of cervical mucus further accommodates 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, clearer, and stickier.

Non-Fertile Stage

During the non-fertile stage of menstruation, the cervical position is lower and the cervical os closes. Vaginal secretions 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 begins to descend into the pelvis in preparation for birth. The role of the cervix at this stage is to provide stable support for the baby as it is pointed head-first toward the cervical canal.

Labor and Childbirth

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

In the process of effacement, the internal and external os come closer together. As effacement and dilation progress, the doctor or midwife uses the extent of the cervical opening to help make decisions about labor management.

  • During early labor, the uterine contractions become strong and regular and the cervical dilates to more than 2 inches (5 centimeters).
  • During active labor, the cervix opens to the full dilation of 4 inches (10 centimeters); this is when pushing begins to deliver the baby through the birth canal.
  • 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 doughnut with a hole or indentation in the middle.

If you are 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.
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Article Sources
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  1. National Cancer Institute. Anatomy of the female pelvis.

  2. 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

  3. 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

  4. Bialy A, Wray AA. Gynecologic examination. In: StatPearls [Internet]. StatPearls Publishing; updated August 24, 2020.

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