Effacement in pregnancy refers to the thinning and shortening of your cervix as your body prepares for labor. The cervix starts out as a firm, cylindrical structure about 3 to 4 centimeters long. As labor approaches, it gradually thins until it’s as flat as a sheet of paper. Effacement is measured in percentages, from 0% (no thinning at all) to 100% (completely thinned out), and reaching 100% is one of two requirements for a vaginal delivery.
How Effacement Works
For most of pregnancy, the cervix acts as a sealed gateway between the uterus and the birth canal. It stays firm, thick, and closed to protect the baby. As your body starts gearing up for delivery, hormones soften the cervical tissue in a process sometimes called “ripening.” Contractions, along with the pressure of the baby’s head pressing down, gradually pull the cervix upward and flatten it into the lower part of the uterus.
Think of the cervix like the neck of a turtleneck sweater. Early in pregnancy it’s long and narrow. During effacement, it gets pulled wider and thinner until it blends into the rest of the fabric. At 0% effaced, the cervix is still its full length. At 50%, it’s about half as thick as it started. At 100%, there’s essentially no cervical length left. Your cervix must reach 100% effacement and 10 centimeters of dilation before a vaginal delivery can happen.
Effacement vs. Dilation
Effacement and dilation are two separate processes that happen together during labor, but they describe different things. Effacement is how thin the cervix gets. Dilation is how wide the cervical opening gets, measured in centimeters from 0 to 10. Both need to be complete before delivery.
For first-time mothers, effacement often happens before significant dilation begins. The cervix may thin considerably while still being only a centimeter or two open. For people who have given birth before, effacement and dilation tend to happen at the same time, which is one reason subsequent labors are often shorter.
How Your Provider Checks Effacement
Effacement is assessed during a cervical exam, where your provider uses their fingers to feel the thickness and length of the cervix. The measurement is somewhat subjective since it’s based on feel rather than a precise instrument. Two different providers might estimate slightly different percentages, so small differences between visits aren’t always meaningful.
Effacement is also one component of the Bishop score, a system used to gauge how ready the cervix is for labor. In that scoring system, minimal effacement (0% to 30%) earns zero points, while 80% or more earns the highest score of 3 points. A higher total Bishop score suggests the cervix is more favorable for labor, which matters if induction is being considered.
In some cases, cervical length is measured more precisely with ultrasound rather than a manual exam. This is especially common earlier in pregnancy when providers are screening for preterm labor risk rather than checking labor progress.
When Effacement Typically Starts
There’s no single week when effacement is “supposed to” begin. Some people start showing early signs of effacement weeks before their due date, while others don’t efface noticeably until active labor kicks in. Being partially effaced at a routine late-pregnancy checkup doesn’t mean labor is imminent. Likewise, having no effacement at 38 or 39 weeks doesn’t mean anything is wrong.
The pace varies enormously from person to person. Some people walk around at 50% effaced for weeks. Others go from 0% to fully effaced in a matter of hours once labor begins.
What Effacement Feels Like
You can’t feel your cervix thinning directly. What you may notice are indirect signs that your body is preparing: increased pelvic pressure as the baby drops lower, more frequent or intense Braxton Hicks contractions, a change in vaginal discharge, or the loss of the mucus plug (a thick clump of mucus that sealed the cervical opening during pregnancy). Some people also feel a dull ache in the lower back or menstrual-like cramping as the cervix begins to change.
None of these signs reliably tell you your exact percentage of effacement, though. The only way to know is through a cervical exam.
When Early Effacement Is a Concern
Effacement that happens too early in pregnancy can signal a risk for preterm birth. Providers pay close attention to cervical length in the second trimester, particularly for people with a history of preterm delivery or cervical procedures. If ultrasound shows the cervix has shortened to less than 29 millimeters but is still above 25 millimeters, your provider will likely monitor it more closely with repeat ultrasounds.
A cervical length of 25 millimeters or less before 24 weeks is more concerning. At that point, a procedure called cervical cerclage (a stitch placed in the cervix to help keep it closed) may be recommended. Progesterone supplements are another option for people whose cervix measures 25 millimeters or shorter, as they can help reduce the chance of preterm delivery.