What Happens in a Cervical Dilation?

Reviewed on 3/24/2021

Cervical dilation

Pregnancy Labor
Dilation of the cervix means widening of the cervix that occurs normally at the end of pregnancy to allow the passage of the baby through the birth canal.

The cervix is also called the mouth of the womb. It is a cylindrical structure (made up of connective tissue fibers and muscles) connecting the uterus with the vagina. The cervix has two main parts: ectocervix and endocervix. The ectocervix is the lower part of the cervix projecting into the vagina. It is felt during an examination through the vagina (per-vaginal [PV] examination). The opening of the ectocervix is called the external os. The endocervix is the upper part of the cervix. It is lined by mucus-producing cells. The endocervix opens into the uterus, and this opening is called internal os. The lining cells of the ectocervix and endocervix are different, and the zone where these two meet is called the transition zone.

Dilation of the cervix means widening of the cervix that occurs normally at the end of pregnancy to allow the passage of the baby through the birth canal. The external os or outer opening of the cervix normally remains closed and opens slightly during menstrual periods to allow the passage of blood and other secretions through the vagina.

Changes in the positions, thickness, width, length, and consistency of the cervix are important predictors of the onset of labor.

  • A sign that a woman will go into labor is the softening of the cervix. The consistency of the cervix changes from a firm like the tip of the nose to soft like the lips.
  • The position of the cervix changes from being behind the baby’s head to forward on top of their head. The cervix moves closer to the opening of the vagina.
  • The walls of the cervix also become thin. This is called the effacement of the cervix. A non-pregnant woman’s cervix is generally 1.5- to 2-inch thick. A 50% effaced cervix is around an inch thick. A 100% effaced cervix becomes very thin, almost like paper.
  • The cervix widens to mark the onset of labor. Generally, the dilation of the cervix is the last thing that occurs.

Three main stages of labor

Cervical dilation starts gradually. The woman may observe the passage of the mucus through the vagina. This is called the mucus plug, which sits inside the cervix to protect the baby from infections. Labor is divided into three main stages:

  1. Stage one involves the dilation of the cervix.
  2. Stage two involves the birth of the baby.
  3. Stage three involves the delivery of the placenta (the connecting tissue between the mother and the baby).

Stage one begins with the start of uterine (womb) contractions. It is the longest stage of labor (lasts for up to 19 hours in first-time moms and up to 14 hours in women who have already been pregnant). The cervix gradually dilates during this stage of labor. It is divided into three phases: latent, active, and transition.

  • Latent phase: It involves the contractions increasing in strength, regularity, and frequency. The cervix becomes effaced (thinned). It may vary in duration from a few days to hours. The membranes (bag of waters) may break in the early- to mid-portion of this stage of labor. The cervix gradually starts dilating from just a dimple to 2-3 cm (almost the width of the penny).
  • Active phase: Cervical dilation occurs rapidly during this stage. The cervix may dilate up to 9 cm (almost the size of an apple) during the active phase. This phase lasts almost 5 hours in first-time mothers and around 2 hours in women who have previously given birth.
  • Transition phase: The cervix continues to dilate, although at a slower pace than the active phase. The maximum dilation of the cervix is 10 cm (or almost the size of a bagel) during the transition phase. This phase also involves the rapid passage of the baby lower into the pelvis and deeper into the birth canal.

Your doctor keeps a record of the changes in the cervix along with other recordings (fetal heart rate and mother’s vitals) as you go into labor through a chart called a partogram. The progressive dilation of the cervix gives a little reassurance that the labor is progressing well. In some women, the cervix may dilate (open) too early during pregnancy. This is called cervical incompetence or insufficiency. It can cause problems such as miscarriage and premature delivery of the baby.

The cervix may be dilated artificially in the first trimester of pregnancy to induce abortion for medical or personal reasons. It may be done by medications or instruments called dilators.

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