As labor nears, the cervix begins to thin or stretch (efface) and open (dilate) to prepare for the passage of the baby through the birth canal (vagina). How fast the cervix thins and opens varies from woman to woman. In some women, the cervix may efface and dilate slowly over a period of weeks. A first-time mother often will not dilate until active labor begins.

Late in your pregnancy, your health professional may manually check (wearing sterile gloves) how much your cervix has effaced and dilated.

Effacement

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.

See a picture of cervical effacement.

Throughout your pregnancy, your cervix has been closed and protected by a plug of mucus. 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.

Dilatation

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.