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The term effacement is most often used to describe the process the cervix goes through to prepare for childbirth. The cervix is located at the bottom of the uterus, where it extends into the vagina. Normally the cervix is shaped like a small, squat cylinder with a central opening that remains tightly closed the majority of time. During menstruation, the cervix opens slightly, called dilation, to allow menstrual fluid to pass through. Effacement of the cervix means it is flattening and thinning out to allow even more dilation prior to childbirth, so that the fetus may pass through to the vaginal canal.
Effacement of the cervix is a gradual process. Towards the end of pregnancy, many women experience Braxton Hicks contractions. These contractions are sometimes referred to as false labor, but they are thought to be a component of effacement. Usually painless, and sometimes imperceptible, Braxton Hicks contractions cause the uterus to tighten for a few minutes at a time. The contractions, in turn, trigger the cervix to begin the thinning process of effacement.
Prior to effacement, when the cervix is thick and tight, it is considered to be 0% effaced. By the time the fetus is ready to pass through the vaginal canal, the cervix is paper-thin and 100% effaced. The portion of the cervix that extends into the vagina is usually about 1 - 1.5 inches (2.54 - 3.8 cm). Therefore, when the cervix is 50% effaced, it has shortened and thinned to about 0.5 - 0.75 inches (1.3 - 1.9 cm). This is measured throughout the first stage of labor by periodic examinations. As effacement progresses, so does dilation, which both occur during the first stage. Once both effacement and dilation are at 100%, the second stage of labor begins and the woman begins to push.
Sometimes women reach the end of pregnancy without the cervix showing any signs of effacement. In this case, a doctor may recommend a vaginal suppository or other treatment to encourage the cervix to soften. This triggers effacement and dilation. However, this type of procedure is usually reserved for women who are considered to be overdue. It's always best to let childbirth occur naturally, but if a woman goes beyond what is considered the appropriate gestational period, doctors may begin inducing pregnancy by first softening the cervix. Most doctors don't consider a fetus to be overdue until the 40th week of pregnancy.