According to WebMD, when a woman's cervix is 90 percent effaced, it has stretched out almost as much as possible in preparation for giving birth. The cervix, the lower section of the uterus, must become thinner shortly before the baby comes.
WebMD explains that the cervix begins to efface at the end of pregnancy when the baby drops lower in the uterus. His head pushes against the cervix, spreading it. After effacement starts, the cervix also starts to open or dilate to allow the baby to pass into the birth canal. While effacement is assessed in percentages, dilation is measured in centimeters. Ten centimeters is considered full dilation.