What Dilation Means in Late Pregnancy

In most healthy, full-term pregnancies, the cervix remains tightly closed until late in the third trimester. At this point, your baby starts to drop down into the pelvis, putting pressure on the cervix and causing it to open up, or dilate, in preparation for labor. Once labor begins, contractions will cause your cervix to dilate fully, allowing your baby to enter the birth canal and, ultimately, be born.

Measuring Dilation

Cervical dilation is measured in centimeters. Your cervix must dilate from zero to 10 centimeters before your baby can be born.

Initially, you won't be able to feel your cervix dilating, but your doctor or midwife can tell you if the process has begun. You will most likely have a vaginal exam between the 35th and 37th weeks of pregnancy as a part of routine group B strep (GBS) screening, during which your practitioner will use a swab to get a sample from your vagina. At this point, she will also check to see if your cervix is dilated.

At 1 cm dilated, the cervical opening is approximately the width of a Cheerio. When it gets to 10 cm, with the help of contractions during active labor, it will be about the width of a bagel.

The big question you may have about being dilated is whether or not that means that your baby is likely to be born soon. While being dilated is certainly a positive step towards labor, in and of itself it is not a sign of labor or even that labor is coming in a certain amount of time.

When Dilation Occurs Too Soon

In around 1 percent to 2 percent of pregnancies, the cervix begins to open well before the pregnancy has gone to term. This is known as having an "incompetent cervix" and is a frequent cause of miscarriages that occur after 20 weeks; premature birth can also occur as a result of an incompetent cervix.

The condition can be caused by damage during surgeries that involve the cervix (such as dilation and curettage or abortion, damage during a previous birth, or congenital abnormalities).

An incompetent cervix is not routinely checked for during pregnancy and therefore is not usually diagnosed until after a second or third-trimester miscarriage has occurred.

If you or your doctor suspect that you may have an incompetent cervix, your doctor can evaluate you before you become pregnant or early in pregnancy. Usually, a diagnosis can be made during a pelvic exam or an ultrasound used to measure the cervical opening or the length of the cervix.

If an incompetent cervix is caught early enough, a procedure known as cerclage, in which the cervix is stitched closed, can prevent pregnancy loss or prematurity.

Cervical Incompetence During Pregnancy

Effacement

In addition to dilating, your cervix will also start to "efface" late in pregnancy. Effacement refers to how thick or thin your cervix is. Prior to pregnancy and into much of the third trimester, your cervix is long and thick. As labor nears, it begins to shorten and thin out.

Effacement is generally measured in percentages by doctors and midwives. When you have an exam, your provider may tell you that your cervix is anywhere from 0 percent to 100 percent effaced. When your cervix is completely effaced it effectively becomes part of the uterine wall, allowing the baby to pass through.

Dilation and the Stages of Labor

The first stage of labor—which is followed by pushing—is typically divided into three phases known as early, active, and transition. Stage one is complete when the cervix has dilated to 10 centimeters.

Cervical Dilation in the First Stage of Labor:

  • Early Phase: The cervix will dilate from 1 cm to 3 cm with mild contractions.
  • Active Phase: The cervix expands from 4 cm to 7 cm and contractions become more intense and regular.
  • Transition Phase: The cervix dilates to 10 cm. Transition ends when the cervix has reached 10 cm and is fully dilated.
The Four Stages of Labor

A Word From Verywell

While being told you're dilated toward the end of your pregnancy is exciting, keep in mind that it doesn't necessarily mean labor is imminent. You can walk around for weeks with your cervix at 1 cm, or go from zero to 10 cm over the course of one day. Talk to your practitioner if you have any questions or concerns about this important precursor to your baby's birth.

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