Cervical Os in Pregnancy and Childbirth

Anatomical model of pregnancy
Photo © Dan Prince/Getty Images

Your body is pretty amazing. When you look at pregnancy start to finish, the way the uterus grows and changes, how your body adapts, and all of the moving parts that are so perfectly timed. It's certainly awe inspiring. One part of the body that is largely ignored until pregnancy, and really, until the talk of giving birth comes up is the cervix and the cervical os. The cervix is the entrance to the uterus. The cervical os is just the opening of the cervix.

If you think of the cervix and a tube, there is both the external and the internal os in the cervix, like a funnel. The internal os is closest to your baby. The external os is the furthest away.

Prior to pregnancy this opening to the uterus allows sperm to enter the uterus during the period of ovulation. It opens and closes ever so slightly, almost imperceptibly to you. This is also what effaces (thins) and dilates (opens) in labor and delivery to allow for the birth of the baby.

If you think of effacement as thinning, image that the internal and external cervical os come closer together. This happens towards the end of pregnancy and well into labor. It is measured in percentages. So a cervix that is 0% effaced has not started thinning yet, and a cervix that is 100% effaced is very thin. Dilation of the cervix is measured in centimeters. 10 centimeters is considered fully dilated. Effacement and dilation are not independent of one another and happen at the same time, though not necessarily at the same pace.

The processes of effacement and dilation occur, for the most part, with the contractions of labor.

"When I first heard the word os, I really thought they were saying oz, as in the Wizard of," remembers one mother. "It wasn't until my first childbirth class where the teacher was going over the anatomy and physiology of the pregnant body that I thought about the cervix or the os in any way shape or form."

"I am embarrassed to admit that until my Lamaze class, I thought that the cervix was a completely different body part. I didn't realize that it was just what you call the opening of the uterus," said Kevin.

When I'm teaching a class or talking to a mother or her partner, I liken the cervix to the lips on your face. When we say lips, we recognize them as their own thing, but we also know that the lips are a part of your mouth, just the doorway. The cervix is no different. This analogy can really help you to understand how the cervix functions as a part of the uterus.

You will generally not pay a lot of attention to your cervix in pregnancy, unless you are experiencing complications. Some complications can include where you are in preterm labor, which may mean that your cervix begins to dilate too soon. You may also hear that you have placenta previa, where part of the cervix is covered with the placenta, blocking your baby's birth.

Also Known As: opening of the cervix, mouth of the womb, doorway to uterus

Common Misspellings: cervical oz, cervix os


The Labor Progress Handbook. Simkin, P and Ancheta, R. Wiley-Blackwell; 2 edition.

Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Sixth Edition.

Preterm Labor. American Congress of Obstetricians and Gynecologists (ACOG) Patient Education. July 2004.

Preterm labor and premature birth, March of Dimes, October 2015.

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