Thinning of the cervix

The lower part of the uterus is called the cervix. Before labor, the cervix is typically 3.5 to 4 centimeters long. As early labor begins, the cervix starts to soften, shorten and thin. That process is called effacement. At this point in labor, you might feel mild contractions that come and go. Or you may feel nothing at all.

Effacement often is talked about in percentages. At 0% effacement, the cervix is at least 2 centimeters long. That means it's still very thick. The cervix must be completely thinned out — 100% effaced — before a vaginal delivery.