Throughout pregnancy, the cervix serves as a barrier to protect a growing fetus during development by remaining firmly closed, but as labor approaches, the cervix effaces and dilates, or shortens and opens, to allow for the passage of the baby through the birth canal, according to BabyZone. Mainly composed of collagen, the cervix is capable of withstanding the growing pressure of a fetus until delivery, as stated in BabyZone.
While it is not possible to feel the changes happening to the cervix as a pregnancy comes to term, a doctor or midwife will check for cervix ripening, or the softening and thinning of the cervix that occurs up to several weeks before labor begins, according to the Mayo Clinic. Thinning, or effacement, is the first step in the ripening of the cervix, and the measure of this is typically stated in percentages with zero percent effacement indicating a thick cervix not ready for delivery and 100 percent effacement indicating that the cervix is completely thinned, according to the Mayo Clinic. Dilation, or opening of the cervix, can begin several weeks before labor but always finishes during active labor and is measured on a scale of 0 to 10 centimeters, according to the Mayo Clinic.