Cervical Os in Pregnancy and Childbirth

Anatomical model of pregnancy
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The cervical os is part of the female reproductive located in the pelvis. It is the opening in the lower part of the cervix between the uterus and vagina. There are three main parts of the cervix:

  • The ectocervix, also known as the cervical canal
  • The internal os, the opening on the innermost side of the canal near the baby
  • The external os, the opening on the outermost side of the canal

The Role of 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 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 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 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 two inches (five centimeters).
  • During the second stage, the cervix will open to the full dilation of four inches (10 centimeters). This is when active pushing begins 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 fingers into the vagina, you can feel the cervix to 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.
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Article Sources

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