What Is the Cervical Os?

A Key Part of the Female Reproductive System

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.

Illustration of a cervix
CFCF / Wikimedia Commons

Anatomy of the Cervix

In order to describe the cervical os, it helps to first review the anatomy of the cervix.

The cervix contains:

  • The ectocervix: The passage between the uterus and vagina
  • The external os: The opening in the center of the ectocervix, also known as the cervical canal
  • The endocervix: Also known as the endocervical canal, the endocervix is a passage that connects the vagina to the uterus.
  • The internal os: The opening into the uterus from the cervix
  • The transformation zone: The border that overlaps the ectocervix and the endocervix

The Role of the Cervical Os in Menstruation

Throughout a woman’s menstrual cycle, the cervical os will open and close in in conjunction with the various phases of menstruation. Depending on the stage, the position of the cervix will constantly 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.

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 infectious agents.

The Role of the Cervical Os in Pregnancy

After conception and the implantation of the fertilized egg in the uterus, the cervical os will change in response to both 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 is pointed in a head-first point toward the cervical canal.

As labor progresses, the cervix will become softer and shorter, and the cervical os will begin to dilate. To accommodate the head of the baby during childbirth, the cervix must open to a diameter of more than four 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 healthcare provider or midwife will use the extent of the cervical opening to help make treatment decisions.

During the first stage of labor, uterine contractions will become strong and regular and the cervix will dilate to more than two inches (five centimeters).

During the second stage, the cervix will open to the full dilation of four inches (10 centimeters). This is when the mother begins active pushing to deliver the baby through the birth canal.

Self-Examination of the Cervical Os

You can locate the cervical os during a simple self-exam. By inserting two (clean) fingers into the vagina, you can feel the cervix toward the back of the vaginal space. 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.

Cervical Stenosis

In some cases, the cervical os will close completely. This is a condition known as cervical stenosis. It is possible to be born with a closed cervix; it can also be a result of uterine surgery, endometriosis, scarring, cysts, or cervical cancer.

Symptoms can include periods that become more irregular or painful. A closed cervix can also cause infertility because sperm can’t travel into the uterus to fertilize an egg.

To diagnose a closed cervix, your gynecologist will need to perform a pelvic examination in order to see your cervix. If your os looks narrow or otherwise appears abnormal she may try to pass a probe through it. If they can’t, you may get a diagnosis of cervical stenosis.

If you have no symptoms or are not trying to become pregnant, treatment is not always necessary for a closed cervix.

But if you’re trying to get pregnant or have painful symptoms, your healthcare provider may recommend using cervical dilators. These are small devices placed in the cervix that slowly expand over time, stretching your cervix.



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2 Sources
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  1. American Pregnancy Association. Signs of Ovulation.

  2. Merck Manual Professional Version. Cervical Stenosis. Updated May 2019.