When you're pregnant, you may learn about body parts you’ve never heard of before. Because let’s be honest: Even if your high school sex-ed class was detailed, how much do you really remember? During pregnancy, you’ll discover firsthand all the amazing things your body innately knows how to do to get baby-ready.

An important player in all this prep work is the cervical os. Here’s what this special region inside the cervix does during pregnancy and delivery, and after childbirth. 

What is the cervical os?

The cervical os (pronounced like “ox” but with an “s”) is part of a woman's cervix. 

First things first: The cervix is the narrow, lower end of the uterus. It looks like a cylinder that’s a little more than 1 to 2 inches long, and it connects the uterus to the vagina. The cervical os describes the openings at the two ends of this canal (called the endocervical canal):

  • The internal cervical os opens into the uterus 
  • The external cervical os opens into the vagina 

During pregnancy, the cervix helps keep the growing baby in the uterus, even as he moves closer to the cervical canal. It also contains the mucus plug — a barrier that protects your baby until he’s ready for the outside world.

In fact, losing your mucus plug can mean that your body is getting ready for childbirth. While some women lose their mucus plug weeks before labor begins, others lose it right as labor starts. 

The cervical os during childbirth

When labor does set in, the cervix begins softening, shortening and thinning (called effacement). The internal and external cervical os will also move closer together. At the same time, the external os starts to open, or dilate.

Throughout labor, your doctor will periodically check your cervix to see how much it's dilated and effaced. It’s just another sign as to how you’re progressing.

If it’s medically necessary, your doctor can use medication to speed labor along. The labor drug Cervidil, for example, can help the cervix dilate and efface if it needs a boost. 

The state of your cervix signals when it's time to push. This second stage of childbirth can begin when your cervix is 100 percent effaced and the external cervical os has dilated to 10 centimeters. It’s officially time for your baby to leave the womb — and finally meet you!

The cervical os after childbirth

Given how much the cervix can change during pregnancy, labor and delivery, you may wonder just how long it will be until the cervix returns to normal after birth.

While childbirth affects every woman's vagina differently, the cervix typically returns to its previous size and shape about six weeks after delivery. The very elastic tissue that makes up the cervix is what allows it to thin, dilate and eventually retract after giving birth — which is why the cervix and the cervical os are among the many biological wonders that make pregnancy possible.