The cervix is the lower, narrow part of the uterus that connects the main body of the uterus to the vagina. During pregnancy, it acts as a secure, closed barrier, maintaining the fetus within the womb. The length of this structure indicates the pregnancy’s stability and its capacity to remain closed until full term. When the cervix prematurely shortens or opens, it raises the risk of spontaneous preterm birth. Monitoring cervical length allows healthcare providers to assess this risk and implement proactive management.
Defining Cervical Length and its Role in Pregnancy
Cervical length is the measurement of the canal running from the internal os (facing the uterus) to the external os (facing the vagina). Throughout a healthy pregnancy, the cervix remains long, firm, and closed, providing mechanical support against the weight of the fetus and amniotic fluid. This rigid structure prevents the early onset of labor.
The process of the cervix softening, shortening, and opening is called effacement and dilation, which normally occurs just before labor begins. If this process starts too early, it leads to preterm labor. Sufficient cervical length reflects the cervix’s ability to resist internal pressure from the growing pregnancy. A shorter length suggests a diminished capacity to maintain the pregnancy.
How Cervical Length is Measured
Cervical length is primarily measured using transvaginal ultrasound (TVUS), which is the most accurate method. The procedure involves inserting a specialized transducer into the vagina, providing a clear, direct view of the cervix without interference from the abdominal wall. This close proximity ensures precise measurement between the internal and external os.
Measurement is typically performed during the mid-trimester, often between 18 and 24 weeks of gestation, as part of routine screening. While transabdominal ultrasound can visualize the cervix, it is less reliable. For example, a full bladder can artificially increase the apparent length on an abdominal scan, leading to an inaccurate reading. For maximum precision, TVUS is the gold standard, often requiring multiple measurements to confirm the shortest, most representative length.
Understanding Normal vs. Short Measurements
In a typical, uncomplicated singleton pregnancy, a cervical length of 30 millimeters (mm) or greater during the mid-trimester is considered normal. This length indicates a low probability of spontaneous preterm birth. The significant threshold for concern is 25 mm, which is the point at which the cervix is officially classified as “short.”
A cervical length of 25 mm or less, especially when measured before 24 weeks, is associated with a substantially increased risk of delivering before 37 weeks. The risk increases progressively as the measurement decreases below this threshold. For example, a cervix measuring 15 mm carries a higher risk than one measuring 24 mm. This direct correlation between numerical length and probability of early delivery dictates the need for closer clinical surveillance and potential intervention.
Clinical Management of a Short Cervix
Once a short cervix is identified, the primary goal of clinical management is to prevent or delay preterm birth through targeted interventions. The most common first-line treatment is the administration of vaginal progesterone, typically given daily from diagnosis until around 36 weeks. Progesterone is a naturally occurring hormone that helps keep the uterus relaxed and may strengthen cervical tissue, reducing the likelihood of premature effacement.
For women with a short cervix and a history of spontaneous preterm birth, or those whose cervix is extremely short (less than 10 mm), a surgical cervical cerclage may be recommended. This involves placing a strong suture around the cervix for mechanical support. The cerclage is typically placed early in the second trimester and removed around 37 weeks. These strategies, including serial monitoring, are designed to extend the pregnancy, allowing the fetus more time to develop.