How Does a Cervix Dilate in Childbirth?

Introduction

The body goes through many changes during labor to adjust to the baby's descent down the birth canal. One of the most important changes is the softening and dilation of the cervix. The cervix is a narrow cylinder shaped opening at the lower portion of the uterus. It connects the uterus and the vagina and during pregnancy the cervix protects the fetus from infection. During pregnancy the cervix thickens considerable closing the narrow gap between the uterus and vagina. But as the body prepares for labor, the cervix thins and sheds much of the lining that kept the growing fetus safe from the outside world.

Latent Phase

The cervix looks like a tube that is approximately 3 to 5 cm long. One portion of the cervix, called the external os, is lodged in the vagina, and the other end of the tube is in the womb, the internal os. During pregnancy a mucous coating called the mucosal plug blocks the opening to prevent bacterial infections within the uterus. As the cervix thins during the preliminary stages of labor, the mucous plug comes out. The thinning of the cervix is the defining function of labor and the loss of the mucous plug marks the beginning of cervical dilation. However, some women may not even notice the loss of the mucous plug and sometimes it is shed as much as a week before contractions actually begin.

Active Phase

The process of cervical dilation is called effacement. A good visual illustration of how the cervix thins is that of a life saver candy dissolving. The candy starts out as a thick cylinder, and if it is submerged in water, it will begin to thin, and over a period of time the candy will shed layer after layer of the inside of the circle until the only thing left is a thin yet wide cylinder. The cervix acts the same way. Once the mucous plug is gone and contractions begin, the lining of the cervix effaces to clear a path for the baby. This is generally referred to as the latent phase of labor, and it continues until the patient is 4 to 6 cm dilated. During this phase of labor, contractions alone help the body dilate, but by 6 cm the baby's head is usually in the birth canal and asserting pressure to dilate more.
Active labor begins at approximately 6 cm dilation. At this time the cervix will begin to dilate much faster and pressure will increase as the baby moves down the birth canal. As the cervix dilates the following 4 cm, many women may get the urge to push or bear down. However, it is very important to wait until the cervix is fully opened to avoid tearing the cervix, which is a very serious complication. Once the cervix dilates to 10 cm, cervix dilation is complete and it is time to push the baby out. At this time the cervix is paper thin and generally cannot be felt because the baby's head is flush with the external os opening.