Above. The transvaginal ultrasound measurement of cervical length leads to insights into the identification and treatment of a number of important clinical conditions, which range from preterm labor to induction of labor, prolonged pregnancies, timing of repeat C-sections, and management of polyhydramnios. The transvaginal ultrasound technique is superior to transabdominal and/or transperineal cervical length for usefulness and accuracy.
Cervical Cerclage and Preterm Delivery
Above. The ACOG Practice Bulletin on Cerclage for the Management of Cervical Insufficiency in 2014 suggests cerclage placement may be indicated in women with a singleton pregnancy who have a history of one or more second-trimester pregnancy losses with painless dilatation and without labor or abruption placenta, history of prior cerclage for painless dilatation, and physical examination of cervical dilatation in the second trimester. In addition, patients with prior spontaneous preterm birth, transvaginal cervical length of < 25 mm before 24 weeks may be candidates.
Cervical Length and Induction of Labor
Above. The cervical length and Bishop score are significant predictors of successful induction of labor. Patients who deliver within 24 hours demonstrate a mean cervical length of 23.1 mm compared to women who deliver after 24 hours, who demonstrate a mean cervical length of 31.3 mm.. An independent predictor of C-section is a cervical length of 20 mm at the time of induction of labor at term.
Cervical Length and Prolonged Pregnancy
Above. The chance for spontaneous labor at 41 weeks is greater in white, parous women and is greater when the maternal body mass index (BMI) is lower and when the sonographic cervix length is shorter. Further, these factors can define the probability of spontaneous labor and the risk of C-section in the week following the 41st week of pregnancy.
In women undergoing induction of labor at 41 3/7 weeks to 42 1/7 weeks, a shorter sonographic cervical length and multiparity predicted the induction to delivery interval and the likelihood of vaginal delivery within 24 hours.
Above. Transvaginal cervical length in women undergoing induction of labor at 41 3/7 to 42 1/7 weeks and the percent of those delivering within 24 hours according to parity.
Cervical Length and Polyhydramnios
Transvaginal Ultrasound: Imaging Considerations
Measurement Technique Summary
Above left. Note the appropriate anatomic landmarks for proper transvaginal ultrasound orientation.
Above right. Note the sequential steps for ultrasound performance.
Above left and right. Follow the imaging sequence as listed above.
Cervical Length Measurement
Above. When the mother is supine, the orientation of the vaginal ultrasound transducer probe in relationship to the position of the maternal feet and head is illustrated as well as the maternal orientation to posterior and anterior.
Above. Standard anatomic landmarks are the bladder, fetal presentation, cervical canal, internal cervical os, external cervical os, and vagina.
Above. Transvaginal ultrasound sagittal view of the cervix with the critical anatomic landmarks illustrated. The gain, zoom, and focal zone should be adjusted to optimize the image and a strict sagittal plane is necessary to image the entire cervix. The cervical canal is imaged horizontally in the middle of the screen, which may not be possible if the cervix is directly anterior.
Above. Once the cervical canal is identified, withdraw the probe slightly. Place the measurement cursors precisely at the closing points of the internal and external cervical os and measure the distance between them. For the internal os, a small triangle is often seen. Place the cursor at the apex of the triangle (the closing point). For the external os, follow the posterior cervical lip until the closing point is identified.
Sources of Error
1. Visualize the entire cervix
Above. The entire cervix is not visualized in this example and the internal cervical os and external cervical os is not well defined. Although the cervical length is probably normal, this is a suboptimal image.
2. Accurate cursor placement
Above. In the image above, the caliper placement is not exact and the distal cervix is not completely visualized, which hampers the recognition of the external cervical os.
3. Excessive probe pressure
Above. Avoid excess pressure on the probe and confirm that the thickness of the anterior and posterior lip of the cervix is the same. In the above example, there is dissimilarity between the thickness of the anterior and posterior cervical lips.
4. Lower uterine segment contractions
Above. Lower uterine segment contractions.
Above. Contractions may lead to an S-shaped canal and asymmetry of the anterior and posterior portions of the cervix.
Anatomic and Technical Pitfalls
In summary, a number of anatomic pitfalls are recognized and include an underdeveloped lower uterine segment hampering the identification of the internal cervical os, focal myometrial contractions, spontaneous cervical change, and endocervical lesions such as polyps. Technical pitfalls include incorrect interpretation due to vaginal probe orientation and cervical distortion by the probe.
Above. Transvaginal cervical length measurement training. A study shows that those with no prior training can adequately perform the transvaginal ultrasound exam after 18 consecutive ultrasound examinations, while those with experience required only 1 practice session to learn the technique.
Above. This image represents a normal transvaginal cervical length with a proper sagittal view, the distinct appearance of the distal cervix, and proper placement of the cursors for measurement.
Above. Cervical shortening on transvaginal ultrasound, and visualization of the amnion and chorion.
Above. Spontaneous change in apparent cervical length over a 2-minute observation interval.
Above. The membranes are filling the upper vagina. The approximate internal cervical os dilatation is 2.5 cm, and the approximate external cervical os dilatation is 5.2 cm.
Above. Breaking of the internal cervical os, and demonstration of the amnion and chorion.
Above. U-shaped funnel measuring 20 mm and cervical length measuring 21 mm.
Above. V-shaped funnel with a short cervix. The distal cervix is poorly visualized in this image.
Above. An example of a transabdominal ultrasound of the cervical length in the presence of a distended maternal bladder. Compared to the transvaginal approach, this technique is not recommended to obtain accurate cervical length.
Above. Short cervix of 9.8 mm with cerclage sutures intact.
Above. This is a transabdominal cerclage suture. This transvaginal ultrasound image shows that the mersilene tape ligature is echogenic and there is no significant cervical shortening. Some ultrasound labs outline the endocervical canal, as illustrated, in the presence of a significantly curved canal, but preference should be given to measurement between cursors, except in extreme cases.