The measurement of cervical length assesses the closed portion of the cervix, the lower, narrow part of the uterus connecting to the vagina. This structure is typically long and firm throughout pregnancy, acting as a natural barrier to protect the developing fetus. Determining its length provides an objective indicator of a pregnancy’s stability. Since the cervix shortens and opens in preparation for labor, an unusually short length early in pregnancy is a significant factor in predicting the risk of preterm delivery.
Why Cervical Length is Measured During Pregnancy
The primary clinical reason for measuring the cervix is to identify pregnancies at risk of spontaneous preterm birth (delivery before 37 weeks of gestation). This information allows for timely intervention to potentially prolong the pregnancy and improve the baby’s outcome. A shorter measurement correlates with a higher likelihood of the cervix opening too early.
Screening is commonly performed during the routine mid-pregnancy ultrasound, typically between 18 and 24 weeks of gestation, which is the most effective window for identifying risk. Providers also focus screening on individuals with specific risk factors. Those with a history of previous spontaneous preterm birth, a prior cervical procedure, or women carrying multiple fetuses are considered higher risk and often require more frequent monitoring.
Using Ultrasound to Determine Cervical Length
The most accurate method for obtaining this measurement is the transvaginal ultrasound (TVUS) examination. Although a transabdominal ultrasound may initially provide a view, the transvaginal approach offers superior image clarity and precision by placing the probe closer to the target structure. This method minimizes distortion and avoids an overly full bladder, which can artificially elongate the visible cervical length.
During the transvaginal procedure, the patient lies on an examination table with an empty or nearly empty bladder, in a position similar to a gynecological exam. A specialized, slender ultrasound probe is gently inserted into the vagina and positioned in the anterior fornix to obtain a clear, sagittal view of the entire cervix. The sonographer must apply only minimal pressure; exerting too much force can compress the tissue and artificially increase the measured length.
For a measurement to be valid, the image must clearly show the internal os (internal opening), the external os (external opening), and the endocervical canal connecting them. The image is magnified so the cervix occupies at least 75% of the screen, allowing for precise placement of the electronic calipers. To account for the dynamic nature of the cervix, the technician typically takes three measurements over three minutes, and the shortest, most stable reading is recorded as the final length.
Interpreting Cervical Length Measurements
Interpretation relies on established clinical thresholds and is considered alongside a patient’s medical history. During the mid-trimester, the average cervical length ranges between 30 and 50 millimeters (3 to 5 centimeters). A length of 25 millimeters or less, measured before 24 weeks, is the standard clinical definition for a “short cervix” and is the point where interventions are considered.
The degree of risk is inversely related to the length of the cervix; a measurement of 15 millimeters carries a significantly higher risk of preterm birth than a measurement of 24 millimeters. However, it is important to understand that a short measurement is a risk indicator, not a definitive prediction that preterm delivery will occur. A substantial percentage of women identified with a short cervix still carry their pregnancy to full term.
Management Options for a Short Cervix
Once a short cervical length is identified, several evidence-based management options are available to reduce the possibility of early delivery. The most common first-line treatment is the daily administration of vaginal progesterone, usually as a suppository. This natural hormone helps maintain uterine quiescence and strengthen cervical tissue, and it is generally continued until 36 weeks of gestation.
For those with a very short cervix or a history of prior spontaneous preterm birth, a surgical procedure called a cervical cerclage may be recommended. This involves placing a strong stitch around the cervix to reinforce it and keep it closed under the pressure of the growing pregnancy. The cerclage is typically placed before the cervix begins to open and is removed near the end of the pregnancy, around 36 weeks, or if labor begins.
Serial monitoring is an important component of management, involving frequent follow-up transvaginal ultrasound scans to check for further shortening or changes in the cervix. This close surveillance allows the provider to assess the effectiveness of current treatment and determine if a more aggressive intervention, such as a cerclage, is warranted. These interventions aim to provide the fetus with the maximum amount of time to develop before birth.