In this article, you’ll learn the important role of the cervix in pregnancy and labor. We’ll cover the cervical changes that take place during each stage of labor. We also give you strategies for picturing the thinning and opening that needs to happen during labor. Being aware of the signs of early dilation can help you reduce your risk of preterm birth, or delivery that happens before the end of your 37th week of pregnancy.
What Is the Cervix?
The cervix is the “neck” or opening at the bottom part of the womb (uterus). It is the gateway between the womb and the vagina. It makes up the lower third of the uterus and controls the movement of substances into and out of the womb. During pregnancy it stays closed. The cervix is made even stronger by a layer of mucus that forms a plug to keep the opening sealed. During labor, the cervix goes through important changes to allow the baby to pass through the birth canal.
What Happens to the Cervix During Labor?
The cervix goes through big changes before and during natural labor. It will dilate—or open—to about 10 cm (about 3.9 inches) wide in order to let the baby’s head and shoulders pass through the vagina. It will also efface—or thin out—to allow for dilation.
Effacement vs. Dilation
Effacement is also called “softening” or “ripening.” Effacement means the thinning out of the cervix. It is measured as a percentage. When the cervix is 0% effaced, it is more than 2 cm (about three-quarters of an inch) thick. When it is 100% effaced, the cervix is as thin as a piece of paper, and dilation can start.
How Cervical Dilation is Measured
During pregnancy, the cervix is fully closed, or 0 cm dilated. When it is fully dilated, the opening is about 10 cm wide. This is large enough for the baby’s head and shoulders to pass through to enter the vagina.
How to Check Cervix Dilation
Doctors and midwives are trained to check cervix dilation with their fingers. They will put on gloves and insert two fingers through the vagina to feel the cervix. They can then estimate the width of the cervix. A dilation check can be uncomfortable for women but should not be painful.
Visualizing Cervical Dilation
During labor, many moms find it helpful to picture the dilation of the cervix to understand the progress the body is making towards a healthy delivery.
A dilated cervix as represented by slices of fruit:
What Causes Cervical Dilation?
Cervical dilation is caused by contractions. When the muscles of the womb contract, they pull the uterus upwards, which stretches the cervix open. During painful labor contractions, it can be helpful to picture the good work that contractions do to help get the cervix opened enough for delivery.
Labor Induction: What if My Cervix Doesn’t Dilate or Efface on its Own?
For some women, the cervix doesn’t efface or dilate naturally. In these cases, a doctor will induce labor by giving certain medicines to help with effacement and dilation. There are several reasons a doctor would induce labor:
- The baby is more than two weeks overdue
- The water has broken (amniotic membranes ruptured) but there are no other signs of labor
- The mother has certain medical conditions, like high blood pressure
- Continuing the pregnancy would risk the health of mother or baby
For more information about labor induction, check out this helpful resource from the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
Signs Your Cervix Is Dilating, and Baby Is Coming
While the process of labor is different for every mother and every pregnancy, there are several key signs and symptoms that may mean that your cervix is dilating, and that baby may not be far away:
- Contractions. Contractions feel like severe menstrual cramps or backache pains. As labor goes on and your cervix dilates, contractions get closer together, last longer, and become more regular in their timing. Learn more about the types of contractions and how to monitor contractions in this blog post.
- Water breaking. When the amniotic sac breaks, amniotic fluid begins to flow from your womb. This is also known as “water breaking” or “ruptured membranes.” Your water can break at the start of labor or during the first stage. The fluid may come out as a gush, or it may flow at a slow trickle. In either case, you should let your doctor know when your water breaks. In some cases, the water does not break on its own, and a doctor will need to break it during labor.
- Passing the mucus plug. The mucus plug blocks the opening of the cervix during most of pregnancy. When the cervix begins to open, it releases the mucus plug. The plug passes through the vagina. It may appear as one thick clump or as a stringy discharge. It is not always a sign that labor is right around the corner, as some moms pass their mucus plug days or weeks before the baby arrives.
- Bloody show. Bloody show is any discharge that is tinged with blood. While it may contain the mucus plug, it is not always the same thing. Any time you notice spotting or bleeding, contact your healthcare provider.
- Feeling new pelvic discomfort. You may feel your baby’s head moving lower down in your belly, which is called lightening. Like some other symptoms, lightening can happen days or even weeks before labor truly starts.
It can be hard to tell when labor is truly starting, or if your symptoms are a normal part of pregnancy. The American College of Obstetricians and Gynecologists has a useful FAQ page about contractions and the other possible signs of labor. The U.S. Department of Health and Human Services offers a wealth of resources about labor and delivery, including information about managing the pain of labor.
If you are in any doubt about whether or not your cervix is dilating and labor has started, contact your doctor right away.
Phases of Dilation/Labor
There are three main stages of labor. They are directly related to the cervix and its level of dilation.
|How Long It May Last (on Average)
|Changes in Cervical Dilation
|What are Contractions Like?
|12-19 hours (for first-time mothers)
|Cervix dilates from 0 to 10 cm
|Mild (early) to severe (transition)
|Early Labor (Latent)
|Cervix dilates from 0 to 6 cm
|May start out mild, come every 5-15 minutes, usually not at regular intervals
|Cervix dilates from 6 to 10 cm
|Longer, stronger, and more painful
Each contraction may last about 45 seconds and come every 3 minutes or so
|Cervix dilates from 8 to 10 cm
|Transition may be the most painful part of the labor process
Contractions are closer together and can last 60-90 seconds
Most women feel pressure and a desire to push
|Can be as short as 20 minutes, or as long as several hours
|Cervix is fully dilated
|Contractions slow down: every 2-5 minutes or so, lasting about a minute
You will push the baby out in time with the contractions
|Usually quick: about 20 minutes (begins 5-30 minutes after baby’s birth)
|Contractions are closer together and not as painful as in earlier stages
Contractions help the placenta separate from the uterus and help you deliver the placenta
For detailed information on the stages of labor, check out this useful page from the March of Dimes.
How Dilation Symptoms and Preterm Birth Risk are Related
If cervical dilation starts before a pregnancy reaches full term (before 37 weeks), it is considered preterm labor. Preterm birth affects about one in ten births, most often in women with no obvious risk factors. Preterm birth, or premature birth, can have serious consequences for babies and families. Learn more about preterm birth.
If your doctor has determined that you are at increased risk of early delivery, your healthcare team will help you prepare for that possibility. To learn how the PreTRM Test for Risk Management can give you and your doctor insight into your risk of preterm birth, check out PreTRM FAQs for Moms.
Paying attention to the possible signs of cervical dilation and other labor symptoms can make the difference in preventing preterm birth.
To learn more about ways you can work with your health care team to prevent premature birth, check out this blog post.
Cervical insufficiency (or cervical incompetence) is a condition that causes the cervix to thin out and open too early. In women with cervical insufficiency, dilation can happen without any contractions. This condition causes about 1% of preterm births. When doctors think a woman may have cervical insufficiency, they use ultrasound pictures to look for signs of early cervical thinning. Treatments include progesterone and cerclage (a procedure to stitch the cervix closed until later in pregnancy). You can learn more about the symptoms, causes, and treatments for an incompetent cervix here.
The cervix plays such an important role in your pregnancy health and labor. It’s helpful to be on the lookout for contractions and other possible signals that your cervical dilation may have begun. If you have any doubts as to whether your cervix may be dilating, seek medical help and contact your healthcare team immediately.
How will you know if you are dilating?
Several common symptoms that may show you that your cervix is dilating are contractions, bloody show, lightening (or the baby’s head moving lower in your belly), and your water breaking.
What week does cervix start to dilate?
Cervical dilation can begin a few days or even weeks before labor starts. In a normal pregnancy, doctors will start to measure cervix dilation at regular visits starting at about 36 weeks of pregnancy. If you have signs of preterm labor, your doctor will check for effacement and dilation earlier.
How can you tell if you are dilated at home?
If you have signs or symptoms of labor, your cervix may have begun to dilate. It’s important to call your doctor right away. It is not recommended to try to measure your cervix dilation at home, since putting anything into the vagina can introduce harmful bacteria.
Talk to your doctor today about the PreTRM Test for your individual risk assessment.