What Is Dilation in Pregnancy?

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Cervical dilation is the opening of the entrance to the uterus that occurs during childbirth. In most pregnancies, the cervix remains long and closed until late in the third trimester. At this point, your baby starts to drop down into the pelvis. This puts pressure on the cervix, causing it to thin out (or efface) and open up (dilate) in preparation for labor.

Once labor begins, contractions and the descent of the baby will usually cause your cervix to dilate fully, allowing your baby to enter the birth canal and, ultimately, be delivered. This may not happen for every woman, even after they go into labor. Cervical dilation may occur naturally or may be induced by a medical professional.

doctor checking dilation on pregnant woman

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How Dilation Is Measured

Progress in labor is measured by how much the cervix has opened and thinned to allow your baby to pass through the vagina.

Cervical dilation is measured in centimeters, and your cervix must dilate from zero to 10 centimeters to enable vaginal delivery. As you get closer to your due date, your cervix may start to dilate without you realizing it. Your doctor or midwife will be able to tell if this is happening during your regular checkups by doing a cervical exam.

At one centimeter dilated, the cervical opening is approximately the width of your index finger. When it gets to 10 centimeters, with the help of contractions during active labor, it will be about the width of a large bagel.

Effacement vs. Dilation

Effacement and dilation are different processes that happen at the same time. Effacement refers to the thinning, softening, and shortening of the cervix. The more effaced the cervix is, the faster dilation happens.

Symptoms

Other than an exam by a healthcare professional, symptoms you’re beginning to dilate include:

  • Losing your mucus plug: During pregnancy, the opening of the cervix is blocked by a thick plug of mucus to prevent bacteria from entering the uterus. During dilation, this plug is loosened. It may come out as one piece or as thick mucus discharge from the vagina. When this occurs, it is an indication that the cervix is beginning to dilate, although not all women will notice this mucus plug being released.
  • Having a bloody show: This usually comes along with the mucus plug and may continue throughout labor, making the mucus tinged pink, red, or brown. Fresh, red blood is usually not associated with dilation, but rather with serious complications such as placental abruption (when the placenta detaches partly or entirely from the wall of the uterus) or placenta previa (when the placenta attaches low within the uterus, covering all or part of the cervix).

Bleeding Warning

Fresh, red blood is usually not associated with dilation, but rather with serious complications such as:

  • Placental abruption: When the placenta detaches partly or entirely from the wall of the uterus
  • Placenta previa: When the placenta attaches low within the uterus, covering all or part of the cervix

If you’re bleeding heavily or seeing signs of blood long before your due date, check in with your doctor right away.

Phases of Dilation

There are three phases of dilation:

  • Early phase: The cervix will dilate from 1 to 3 centimeters with mild contractions.
  • Active phase: The cervix expands from 6 to 10 centimeters, and contractions become more intense and regular.
  • Transition phase: The cervix dilates to 10 centimeters. Transition ends when the cervix has reached 10 centimeters and is fully dilated.
I'm dilated

Alison Czinkota / Verwell

There are three stages of labor:

  • Stage one: When the cervix opens to 10 centimeters dilated
  • Stage two: When the baby moves down through the vagina and is born
  • Stage three: When the placenta (afterbirth) is delivered

How Long Is Stage One of Labor?

Every labor is different, but the first stage of labor is the longest stage. For the first pregnancy, it can last from up to 20 hours. It may be shorter (about 12 hours) for those who’ve already had children.

Complications

Sometimes, there may be medical reasons to speed up the process of dilation and kickstart labor. Medical intervention may be a good idea if:

  • You’re almost two weeks past your due date, and labor has yet to begin
  • Your water has broken, but you aren’t experiencing contractions
  • Your baby isn’t growing at a steady pace
  • There isn’t enough amniotic fluid around your baby
  • You have a medical condition, like diabetes or high blood pressure, that could be dangerous for you or your baby if the pregnancy is allowed to continue

Induced Dilation

Normally, your cervix begins to dilate naturally on its own during labor, but when this doesn’t happen or dilation slows down, your doctor may need to step in to get things moving.

They may use one of the following methods:

  • Mechanical devices: The most popular and successful mechanical approach is using a Foley bulb. Your doctor inserts this balloon-like device into the uterus and fills it with saline. The resulting pressure should inspire the cervix to dilate.
  • Medication: The hormone prostaglandin may be applied onto the cervix or into the vagina. It triggers cervical softening and labor contractions. If it doesn’t work, your doctor may try an oxytocin drip.
  • Membrane stripping: This technique is used if your amniotic sac is unbroken. Your doctor will brush a finger across the membranes attached to the sac, which prompts the uterus to secrete prostaglandin.

When Dilation Happens Too Early

In around 1% of pregnancies, the cervix begins to open well before the pregnancy has gone to term.

What Is Cervical Insufficiency?

Cervical insufficiency—also called incompetent cervix—is when the cervix dilates painlessly without any contractions. When this premature dilation is not detected in time, cervical insufficiency can cause a pregnancy loss or birth of a preterm infant.

Cervical insufficiency can be caused by:

  • Abnormally formed uterus or cervix
  • Short cervix
  • Damaged cervix from previous miscarriage or childbirth
  • Damage during surgeries that involve the cervix (such as a LEEP procedure)

Unfortunately, cervical insufficiency usually has no symptoms in the first affected pregnancy.

The cervix dilates without the individual necessarily noticing any contractions, and then the waters break and the baby is born—sometimes too early to have a chance at survival.

You may have some spotting or bleeding, but usually, by the time the condition is detected, it is too late to stop the preterm birth.

If cervical insufficiency is caught early enough, a procedure known as a cervical cerclage, in which the cervix is stitched closed, can be performed. This treatment may prevent pregnancy loss or preterm birth.

A Word From Verywell

Cervical dilation means that your body may be on its way to preparing for your baby’s arrival. Unfortunately, it’s not a reliable indicator of when the whole process will truly kick into high gear.

During cervical dilation, try to remain patient, keep in close contact with your doctor, and monitor yourself for any other labor symptoms. Call your doctor if you notice changes they haven’t discussed with you before.

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  5. March of Dimes. Medical reasons for inducing labor. Updated September 2018.

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  7. Cleveland Clinic. Incompetent cervix: causes, treatments & tests. Updated October 9, 2019.