Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Cervical dilation is the dilation (opening) of the cervix during childbirth. In the later stages of pregnancy, the cervix may already have opened up to 1-3 cm (or more in rarer circumstances), but during labor, repeated uterine contractions lead to further widening of the cervix to about 6 centimeters. From that point, pressure from the presenting part (head in vertex births or bottom in breech births), along with uterine contractions, will dilate the cervix to 10 centimeters, which is "complete." Cervical dilation is accompanied by effacement, the thinning of the cervix.
General guidelines for cervical dilation:
- Early Labor: 0-3 centimeters
- Active Labor: 4-7 centimeters
- Transition: 8-10 centimeters
- Complete: 10 centimeters. Expulsion of the fetus takes place shortly after this stage is reached (although the mother does not always push right away.)
During pregnancy, the os (opening) of the cervix is blocked by a thick plug of mucus to prevent bacteria from entering the uterus. During dilation, this plug is loosened. It may come out as one piece, or as thick mucous discharge from the vagina. When this occurs, it is an indication that the cervix is beginning to dilate, although not all women will notice this mucus plug being released.
Bloody show is another indication that the cervix is dilating. Bloody show usually comes along with the mucus plug, and may continue throughout labor, making the mucus tinged pink, red or brown. Fresh, red blood is usually not associated with dilation, but rather serious complications such as placental abruption, or placenta previa. Red blood in small quantities often also follows an exam.
Prostaglandins contribute to cervical ripening and dilation. The body produces these naturally. Sometimes prostaglandins in synthesized forms are applied directly to the cervix during labor induction to soften the cervix, though these medications have been linked to an increase in uterine rupture risk especially for those with previous cesarean sections.
The pain experienced during dilation is similar to that of menstruation (although markedly more intense), as period pains are thought to be due to the passing of endometrium through the cervix. Most of the pain during labor is caused by the cervix dilating, not the uterus contracting.