The cervix is the thick, muscular, lower part of the uterus that extends into the vagina, acting as a structural barrier that keeps a pregnancy contained until term. During pregnancy, the length of this structure is regularly monitored because it provides a strong indication of the uterus’s ability to maintain the gestation. Tracking this measurement helps medical providers identify individuals who may be at an increased risk for delivering prematurely. Understanding what constitutes a normal measurement and the methods used for assessment is a crucial step in prenatal care.
Understanding Cervical Length Measurement
Measuring the cervix is a specialized procedure most accurately performed using a transvaginal ultrasound (TVS). This technique involves placing a slender ultrasound probe into the vagina, which allows for a clear and direct image of the cervix. This provides a precise measurement from the internal opening (os) to the external opening. The transvaginal approach is preferred because a transabdominal ultrasound, where the probe is placed on the abdomen, can sometimes give a falsely elongated measurement.
For most of the second trimester, the cervix should be relatively long and closed. At approximately 20 weeks of gestation, the average cervical length is about 40 millimeters (mm). While measurements between 35 mm and 48 mm are generally expected between 16 and 24 weeks, any measurement greater than 30 mm is typically considered within the normal range. The length of the cervix is not static and may exhibit dynamic changes throughout the assessment.
One such dynamic change is known as “funneling,” where the internal os begins to open, and the amniotic membranes start to protrude into the cervical canal, creating a funnel-like shape. This funneling is a sign of premature cervical ripening or softening. Although a very short cervix is often accompanied by funneling, the measurement of the functional, closed portion of the cervix remains the primary focus.
The Link to Preterm Birth Risk
The length of the cervix serves as the single most reliable predictor of whether a pregnant person will experience a spontaneous preterm birth (sPTB), defined as delivery before 37 weeks of gestation. The risk of delivering early increases significantly as the cervical measurement decreases. The clinical threshold that defines a “short cervix” is widely recognized as a length of less than 25 mm before 24 weeks of pregnancy.
When the cervix measures below this 25 mm threshold during the mid-trimester, the probability of an sPTB rises substantially. This measurement is considered particularly abnormal if it is less than 15 mm, which indicates a substantially elevated risk of very early delivery.
Cervical length is typically assessed during the second trimester, often between 18 and 24 weeks of gestation, as this window is the most predictive for identifying risk. Shortening of the cervix during this period suggests a potential failure to maintain the pregnancy to term, a condition sometimes referred to as cervical insufficiency. This early effacement is a sign that the structural support of the uterus is weakening prematurely, which can initiate the events leading to labor.
Interventions for a Short Cervix
When a short cervix (less than 25 mm) is identified, medical intervention can significantly reduce the risk of spontaneous preterm birth. The primary management strategy involves the use of vaginal progesterone. This hormone is typically administered daily, often as a suppository, from the time of diagnosis until around 36 weeks of gestation.
Vaginal progesterone works by helping to relax the uterine muscle and potentially strengthening the cervical tissue, which can prevent further shortening and help maintain the pregnancy. Studies have shown that this treatment can reduce the rate of sPTB in women with a short cervix. For individuals whose cervix measures between 20 mm and 25 mm, providers may opt for enhanced monitoring with serial transvaginal ultrasounds before starting progesterone.
A cervical cerclage is a surgical procedure where a strong, non-absorbable suture is placed around the cervix to provide mechanical support. This procedure is generally reserved for individuals considered to be at a higher risk, such as those with a history of a previous preterm birth or those whose cervix continues to shorten significantly despite progesterone therapy. Cerclage acts as a physical barrier to premature dilation, and when combined with progesterone in appropriate candidates, it offers a strategy for extending the pregnancy duration.