The cervix is the lower, narrow portion of the uterus that connects to the vagina. During pregnancy, its function is to act as a physical barrier, remaining long, firm, and closed to keep the developing fetus secure. This biomechanical structure must withstand increasing pressure as the pregnancy advances. The integrity of this barrier is directly related to the duration of the pregnancy. Premature shortening can lead to complications, so measuring cervical length assesses the risk of delivering a baby too early.
Understanding the Purpose of Cervical Length Measurement
The primary reason for measuring cervical length is to assess the risk of spontaneous preterm birth, defined as delivery occurring before 37 weeks of gestation. A shortened cervix is the most predictive factor for this complication in the second trimester. Identifying this risk allows healthcare providers to implement preventative strategies that can prolong the pregnancy and improve the baby’s outcome.
Screening is not routinely performed for all pregnancies, but it is recommended for specific patient populations. This includes women with a history of prior spontaneous preterm birth, which is the strongest risk factor for recurrence. Screening is also common for those carrying multiples or when a routine transabdominal ultrasound suggests the cervix may be shorter than expected.
Performing the Transvaginal Ultrasound Measurement
Measuring the cervix accurately requires a transvaginal ultrasound (TVUS), which is considered the standard technique. Unlike a transabdominal scan, TVUS involves inserting a specialized transducer a short distance into the vagina. This positioning allows the probe to rest closer to the cervix, providing a clearer and less distorted image for measurement.
The patient is typically positioned on her back with her knees bent, similar to a gynecological exam, and should have an empty bladder to reduce pressure on the cervix. The sonographer must obtain a precise sagittal view, ensuring the entire cervical canal, from the internal opening (internal os) to the external opening (external os), is visible. It is important to apply minimal pressure with the probe, as excessive force can artificially lengthen the cervix and skew the result.
Once the optimal view is captured, electronic calipers are placed at the internal and external os to measure the distance in millimeters. The sonographer often takes a series of measurements, sometimes applying light pressure to the top of the uterus to check for dynamic changes like funneling. Funneling occurs when the membranes begin to protrude into the internal os, indicating a potential weakening. The shortest and most stable measurement obtained is the value reported to the physician.
Interpreting Cervical Length Measurements and Associated Risk
The length of the cervix normally measures between 30 and 50 millimeters (3.0 to 5.0 cm) during the middle of a pregnancy. Risk is categorized using specific cutoff points established through large-scale clinical studies. A measurement below 25 millimeters before 24 weeks of gestation is the universally accepted threshold for defining a short cervix and significantly increases the risk of preterm delivery.
As the measurement decreases below this 25-millimeter cutoff, the likelihood of premature birth rises proportionally. For instance, a cervix measuring between 20 and 25 millimeters indicates an elevated risk, but a measurement below 15 millimeters is associated with a substantially higher probability of very early delivery. This inverse relationship between length and risk helps guide the urgency and type of medical intervention considered.
Medical Interventions for a Short Cervix
When a short cervix is identified, primary medical interventions focus on strengthening the cervical tissue or mechanically supporting it. Progesterone supplementation is often the first-line treatment, particularly for women with no prior history of preterm birth who are found to have a short cervix. This hormone is typically administered daily as a vaginal suppository or gel, relaxing the uterus and stabilizing the cervical tissue to prevent further shortening.
For women at higher risk, such as those with a history of spontaneous preterm delivery or a very short cervix, a surgical procedure called a cervical cerclage may be recommended. This intervention involves placing a strong suture around the cervix to reinforce the muscular barrier and keep the internal os closed. The cerclage is usually placed before 24 weeks and removed around 36 weeks to allow for labor and delivery.