What Is the Cervical Os?

The cervical os is part of the cervix, which is in the lower part of the uterus. It connects the main part of the uterus to the vagina.

The cervix is about and inch or so in length but can vary in length and width during a woman's lifetime.

This article discusses the anatomy of the cervix and location of the cervical os. It also covers the role of the cervical os in menstruation and pregnancy.

Anatomy of the Cervix

To describe the cervical os, it helps to first review the anatomy of the cervix. The cervix contains:

• Ectocervix: The outer part of the cervix that can be seen by the doctor in a pelvic exam
• External os: The opening in the center of the ectocervix
• Endocervix: A passage that connects the vagina to the uterus, also known as the endocervical canal
• Internal os: The opening into the uterus from the cervix
• Transformation zone: The border that overlaps the ectocervix and the endocervix

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.

Cervical Os and Menstruation

Throughout the menstrual cycle, the cervical os will open and close in conjunction with the various phases of menstruation. Depending on the stage, the position of the cervix changes, sometimes higher and sometimes lower.

During ovulation, the cervix is high and positioned closer to the top of the vagina. This change of position allows the cervical os to open more readily to allow sperm to enter. When ovulating, the cervical os is open and relaxed, and the surrounding cervix is high and soft with a feeling similar to pursed lips.

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 (basic) one. The mucus becomes thinner and clearer to ensure the sperm can make their way to the cervical os.

During the non-fertile stage of menstruation, the cervix is low and firm and the cervical os closes. Vaginal secretions become thicker and more acidic to protect against bacteria and other infectious agents.

Cervical Os and Pregnancy

After conception and the implantation of the fertilized egg in the uterus, the cervical os changes. This change is in response to both the stage of pregnancy and the growth of the developing fetus.

As the pregnancy enters the third trimester, the fetus begins to descend in the uterus in preparation for birth. The cervix provides stable support for the baby’s head at this stage. The baby is usually in a head-first position toward the cervical canal.

As labor progresses, the cervix becomes softer and shorter, and the cervical os begins to dilate or open. To accommodate the head of the baby during childbirth, the cervix 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 healthcare provider or midwife use the extent of the cervical opening to help make treatment decisions.

During the first stage of labor, uterine contractions become strong and regular, and the cervix dilates to more than 4 inches (10 centimeters).

During the second stage, the cervix opens to the full dilation of 4 inches (10 centimeters). This is when the mother begins actively pushing to deliver the baby through the birth canal.

What Is a Closed Cervix?

In some cases, the cervical os will get abnormally narrow or 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 needs to perform a pelvic examination to see your cervix. If your os looks narrow or otherwise appears abnormal, they may try to pass a probe through it. If they can’t, you may get a diagnosis of cervical stenosis.

If you don't have symptoms or are not trying to become pregnant, treatment is not always necessary for cervical stenosis.

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.

Summary

The cervical os is the opening in the cervix at each end of the endocervical canal. The external os is near the vagina, and the internal os is near the uterus.

During your menstrual cycle, the cervical os opens more readily during ovulation to allow sperm to enter. When pregnant, the cervical os will open (dilate) during labor to allow the baby to come through the birth canal.

A condition called cervical stenosis, or closed cervix, can cause painful periods. It may also cause infertility because the sperm can't reach the egg. To correct this, a cervical dilator can help to open the cervix slowly over time.

A Word from Verywell

The cervix plays a critical role in the female reproductive system. While a small part of the uterus, it affects fertility, allows for vaginal delivery of a baby, and changes throughout a woman's lifetime.