What is the Internal Os?

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  • Written By: Bobbie Fredericks
  • Edited By: Jenn Walker
  • Last Modified Date: 04 December 2018
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The internal os is part of the cervix, which forms the neck of the uterus, part of the female reproductive system. Running through the cervix is an opening called the os, with the end that opens into the vagina called the external os, and the end that opens into the uterus called the internal os. The cervix allows for the passage of menstrual blood and semen, inhibits the passage of harmful organisms into the uterine cavity, and forms the birth canal, which a fetus passes through to the outside world.

The internal portion of the cervix and uterine cavity are covered with stratified squamous epithelium. This layer secretes mucus, which serves several purposes. Around the time of ovulation, the mucus thins out, allowing sperm to move easily through the vagina into the uterus while providing protection against the harsh, acidic vaginal environment. At all other times, the cervical mucus is thick, preventing the movement of sperm and harmful organisms.

Under the epithelial lining, the cervix is muscular. The smooth cervical muscles allow it to dilate and contract. The internal os is about 0.27 inches (about 7 millimeters) wide in a non-pregnant woman of childbearing age. The most extreme dilation occurs during childbirth, when the opening reaches 3.93 inches (about 10 centimeters).


The os is usually firmly closed in non-pregnant females who have never given birth, and slightly open in women who have given birth. The closure helps keep bacteria from the vagina from entering the uterus. During menstruation, the os opens slightly to allow the uterine lining to pass. It also opens during ovulation, to allow sperm to pass through, and it dilates during childbirth to allow the baby to pass through.

During pregnancy, the internal opening sometimes begins to dilate before the fetus has reached term. This is called funneling, and it means the cervix is incompetent. An incompetent cervix cannot support the fetus as it gets bigger, and most of these pregnancies are lost before the fetus is able to survive outside of the womb. Funneling can be diagnosed using an internal ultrasound. Bed rest may be used in an attempt to bring the pregnancy to term, or the external portion of the os may be stitched.

Placenta previa is a complication of pregnancy in which the placenta partially or completely covers the internal os. If labor occurs, the placenta can tear away from the uterus prematurely, causing profuse bleeding. Most cases are diagnosed early, and a cesarean delivery scheduled. Ideally, the placenta should lie at least 0.79 inches (about 2 centimeters) away from the opening.


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Post 3

It's really crazy how much the internal os and the rest of the cervix dilates during childbirth. There is a big difference between 10 centimeters and just a few millimeters! And then afterwards it mostly goes back to the way it was before. Amazing!

I have to wonder about the internal os after childbirth though. Are women who have had children slightly prone to more bacterial infections because their internal os is open a little bit? Or does that small opening not even matter?

Post 2

@JessicaLynn - The first time I heard the term "incompetent cervix" it made me giggle a little. I can only imagine how I would feel if I had a pelvic sonogram and the doctor turned to me and said, "You have an incompetent cervix!" It definitely makes it sound like the cervix has a mind of its own!

Although, it sounds like it kind of does. I never realized how amazing that whole part of the body is until I read this article. It opens more or less at certain times and stops harmful bacteria from entering the body through the reproductive system! I think the cervix is definitely an under-appreciated body part.

Post 1

Is it just me, or does the term "incompetent cervix" sound a bit insulting? After all, no one wants to hear that their reproductive system is incompetent. It almost sounds like your system isn't trying hard enough on purpose!

Anyway, I know someone who had an incompetent cervix. Her internal cervical os started dilating way before her pregnancy was over. Her doctor put her on bed rest, which was very difficult. Most of us like to relax in bed, but it becomes way less fun when you have to! Luckily in my friend's case the bed rest worked, and she gave birth to a healthy baby.

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