As labour nears, the cervix may begin to thin or stretch and open to prepare for the passage of the baby through the vagina (birth canal). How fast the cervix thins and opens varies from woman to woman. In some pregnant women, the cervix may start to efface and dilate slowly over a period of weeks. A first-time mother often will not dilate until active labor begins.
This medical animation shows a time lapse view of labor and delivery during normal vaginal birth. Also shown in detail is dilatation (dilation/dilating) and effacement (thinning) of the cervix occurring prior to birth.
Effacement of the Cervix: In most cases, the baby’s head is down, so the doctor continues with his exam and measures the effacement of the cervix (or the thinness of the cervix). Effacement is measured in percentages. In a normal, non-laboring woman the cervix is 3-4 centimeters long, which is 0% effacement. If a woman is in labor, then the cervix thins. As it thins, it starts to stretch and becomes incorporated into the lower part of the uterus. For example, if the doctor determines the cervix is about 2 cm. thick, then the effacement is approximately 50%.
Dilation of the Cervix: After the cervix begins to efface, it will also begin to open (cervical dilatation). Cervical dilatation is expressed in centimeters from 0 to 10. Zero means that the cervix is closed, and 10 means that it is completely dilated. Your cervix must be completely dilated before you can begin the pushing stage. Occasionally, the baby will be breech – that is, the doctor will feel the baby’s butt or feet. If this condition is diagnosed, the patient will probably be advised to have a c-section, or the doctor may try to turn the baby to the head-down position.