The cervix is the lower, narrow part of the uterus that connects it to the vagina. Throughout gestation, it remains long, firm, and closed to protect the developing pregnancy. Measuring the cervix is an important medical procedure used to monitor health, assess the risk for preterm delivery, and track the progression of labor. This assessment focuses on three aspects: the width of the opening (dilation), the thinning of the tissue (effacement), and the overall length of the structure.
Assessing Labor Progress: Dilation and Effacement
Dilation and effacement are typically measured when a patient is near term or has entered the active phase of labor. This assessment uses a manual internal examination, where a healthcare professional inserts two gloved, sterile fingers into the vagina. The examination estimates the changes in the cervix that allow the baby to pass through the birth canal.
Dilation refers to the opening of the cervix and is measured in centimeters (cm), progressing from zero to a maximum of ten. A cervix that is completely closed is 0 cm dilated, while 10 cm indicates the cervix is fully open. Ten centimeters marks the point where the second stage of labor, or pushing, can begin.
Effacement describes the process of the cervix thinning and shortening from its normal, thick state. This thinning is measured as a percentage, ranging from 0% (no thinning) to 100% (paper-thin). The two processes must both be complete for full readiness for vaginal delivery.
In a first-time pregnancy, the cervix often effaces significantly before it begins to dilate. In subsequent pregnancies, dilation may occur earlier alongside effacement. The results of this manual check are reported together, such as “3 cm dilated and 50% effaced,” to describe the current status of labor progress.
Assessing Preterm Risk: Measuring Cervical Length
Measuring cervical length is performed earlier in pregnancy, primarily to assess the risk of spontaneous preterm birth. Unlike the manual check for dilation and effacement, this measurement uses a technological method. The standard assessment method is the Transvaginal Ultrasound (TVU).
The TVU involves placing a specialized ultrasound probe into the vagina, providing a clear image of the cervix. This technique measures the cervical canal’s closed portion in millimeters (mm). It is typically performed during the second trimester, often between 18 and 24 weeks of gestation.
A normal cervical length in the second trimester is generally 30 to 35 mm, indicating a low risk for early delivery. A measurement of 25 mm or less defines a “short cervix.” This threshold is used because a shorter length signals a higher risk of spontaneous preterm birth.
The presence of a short cervix prompts medical interventions, such as administering vaginal progesterone or placing a cerclage. A cerclage is a stitch placed around the cervix to reinforce it. This screening allows preventative measures to be put in place before the cervix begins to open.
Understanding the Results
Interpreting cervical measurements provides distinct information about the stage of pregnancy and the outlook for delivery. A measurement of 10 cm dilation and 100% effacement signals that the first stage of labor is complete. These manual measurements are the most direct indicators of a patient’s progress during active labor.
Cervical length measurement (25 mm or less) is an indicator of risk rather than an immediate sign of labor. This length alerts the medical team to an increased chance of the cervix shortening further and opening prematurely. The shorter the length drops below 25 mm, the higher the risk for a very early delivery.
Healthcare providers may use the Bishop score to consolidate manual examination results and predict the likelihood of successful labor induction. This score incorporates dilation, effacement, cervical consistency, position, and the baby’s position in the pelvis. A higher Bishop score, generally 8 or more on the 13-point scale, suggests the cervix is ready for labor or successful induction.
Professional Guidance and Self-Monitoring
Cervical measurement requires the expertise of a trained healthcare professional, such as a doctor, midwife, or nurse. The manual check requires training to accurately estimate the centimeter and percentage measurements. The transvaginal ultrasound also requires specialized equipment and clinical training to ensure correct image capture and measurement.
Patients should avoid attempting to perform their own cervical checks at home. This carries a risk of introducing bacteria into the vaginal canal, potentially leading to an infection that may compromise the pregnancy. Self-monitoring without proper training can also lead to inaccurate readings, causing unnecessary anxiety or a false sense of security.