When labor nears, a woman’s cervix starts to stretch or thin (efface) and open (dilate) in order to provide passage for the baby via the vagina (birth canal). As the baby’s head moves towards the pelvis, the head pushes against the cervix and causes it to relax and efface.Throughout a woman’s pregnancy, the cervix is closed and protected by a block of mucus. As soon as the cervix is effaced, the mucus block loosens and flows out of the vagina. Blood may be present in the mucus. Effacement is expressed in percentage. For instance, there is 100% effacement if the cervix is fully thinned.