As your due date gets closer, your cervix will begin to stretch (efface) and open (dilate) to prepare for your baby’s birth. In some women, particularly if this is their second or third pregnancy, the cervix may efface and dilate slowly over a period of weeks. A first-time mother often will not dilate until active labor begins.
Throughout your pregnancy your cervix has been tightly closed and protected by a plug of mucus. As the baby’s head drops down into the pelvis, it pushes against the cervix and causes the cervix to relax and thin out, or efface. When the cervix effaces, the mucus plug is loosened and passes out of the vagina. The mucus may be tinged with blood. This passage of the mucus plug is called show” or “bloody show.” You may or may not notice when the mucus plug passes.
Effacement is described as a percentage. For example, if your cervix is not effaced at all, it is 0% effaced. If the cervix has completely thinned, it is 100% effaced.
After the cervix begins to efface, it will also begin to open (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. You will be ready to push when you are 10 centimenters.
Your health care provider will begin checking your cervix for effacement and dilatation during the final weeks of pregancy, and during your labor to see how it’s progressing.”