Cervix Transvaginal Ultrasound (TVUS)
Acknowledgements and Editorial Advice: Helen Feltovich, MD, MS; Maternal-Fetal Medicine specialist, Intermountain Healthcare and Associate Research Professor, Medical Physics, University of Wisconsin-Madison.
Image Review: Krista Wald, RDMS, Staff sonographer, Perinatology Clinics, and Paula Wickham, RDMS, Head sonographer, Perinatology Clinics: The Mother Baby Center (Allina Health and Children’s Hospitals and Clinics, Minnesota).
This chapter is devoted to transvaginal ultrasound (TVUS) of the cervix. It includes an overview of ‘Preterm Labor’ (PTL), and cervical length (CL) in the ‘Prediction’ of Preterm Labor. In addition, ‘Progesterone Supplementation’ for the prevention of preterm delivery (PTD) and the use of cervical length (CL) for the management of ‘Threatened PTL’ (Preterm Labor) are reviewed. ‘Other Uses’ of TVUS are briefly reviewed (cervical cerclage, induction of labor, delivery timing of repeat C-sections and management of prolonged pregnancies and polyhdramnios). Detailed ‘Imaging Considerations’ are presented and ‘Images’ and ‘Transvaginal Ultrasound Videos’ are included.
Information Cervical Length
Transvaginal Cervical Length
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.
The prevention of preterm delivery is one of the most important issues facing contemporary obstetrics. Above. Defined as delivery occurring at less than 37 weeks, the worldwide incidence of preterm birth is 9.6% , while the global incidence is about 12%*. Higher rates are reported for Africa and North America and lower rates for Europe. In addition, preterm delivery is responsible for higher rates of neonatal death and morbidity which extend into later life (bronchopulmonary dysplasia, brain injury)  and result in increased health care costs.
About 30 to 35% of preterm births are related to medical indications, 40 to 45 % follow spontaneous premature labor and 20 to 25% follow premature rupture of the membranes, while about 40% of preterm births occur at 33 weeks or less gestational age. 
Preterm Labor Causation
Spontaneous preterm birth is multifactorial and poorly understood. Regardless of etiology, its varying pathways lead to gross cervical changes commonly associated with early delivery, such as shortening, softening and dilatation.  Solving the problem of preterm birth will require elucidation of molecular and biochemical events underlying the microstructural and biochemical changes that promote preterm birth. 
Strategies to Prevent Preterm Labor
Above. A number of strategies are proposed to prevent preterm birth. Preventive measures and interventions are possible. For example, efforts to reduce the frequency of multifetal pregnancies by reproductive health care practitioners and efforts to reduce induction of labor before 39 weeks are associated with reduced preterm births rates, while interventions such as progesterone supplementation and cervical cerclage also reduce the frequency of preterm birth.  Other strategies include tobacco control, fertility control and dedicated prevention clinics. 
Cervical Length and Prediction: Preterm Labor
Above. The detection of a short cervix by transvaginal ultrasound at 18 to 24 weeks gestation is the most powerful predictor of spontaneous preterm birth.  In a systematic review among asymptomatic women at increased risk for preterm birth, the most common transvaginal ultrasound cervical cutoff is <25 mm to predict preterm birth at < 35 weeks gestation.  However, other critical cutoff values have been proposed to select women for certain therapeutic interventions.
Above. The cervical length during a normal pregnancy is >30 mm., while a cervical length measured between 14 and 24 weeks gestation of 15 mm. or less is associated with a 50% chance of preterm delivery at less than 33 weeks gestation.  In women with a prior spontaneous preterm birth, the shorter the cervix the more likely recurrent preterm birth occurs and when the cervix is <10 mm, there is a 90% chance of preterm birth before 35 weeks. 
Cervical Length and Twins
Is second trimester cervical length (CL) a risk indicator for Cesarean delivery in twins?
In this study 311 women with a twin gestation attempted vaginal delivery after 34 weeks. Second trimester cervical length was measured in these women to determine if there were an association with the risk of emergency Cesarean section. In nulliparous women, there was no relationship between second-trimester CL and Cesarean delivery. 
Above. Hassan and associates reported a 45% reduction in the rate of preterm singleton births before 33 weeks in women who received vaginal progesterone and had a sonographic short cervix of 10 to 20 mm. at 19 to 23 6/7 weeks.  A meta-analysis of 5 trials suggests that the administration of vaginal progesterone to women with a cervix of ≤ 25 mm. is associated with a reduction in preterm birth at <28 weeks, and <33 weeks, <35 weeks as well as fewer neonatal complications.  The natural vaginal progesterone daily dose among the studies was between 90 mg and 100 mg. Presently, formulations and administration routes have not been vigorously tested.
Routine Cervical Length Screening and Progesterone to Prevent Preterm Birth
Above. The ACOG Practice Bulletin on Prediction and Prevention of Preterm Birth in 2012 (confirmed in 2014) recommends that progesterone supplementation should be offered to women at 16 to 24 weeks with a previous singleton preterm birth irrespective of cervical length and in women who incidentally are noted to have a short cervix of ≤ 20 mm. at or before 24 weeks.  While the Bulletin does not mandate universal screening, universal screening “may be considered”. This opinion is consistent with support for universal screening by others.  While the option to consider universal screening is open, Iams suggests that programs pursing this path must adhere to cost effectiveness, efficiency, and quality standards to ensure accurate measurements and treatment.  The Cervical Length Education and Review (CLEAR) entity is a credentialing program for cervical length measurement offered through the Perinatal Quality Foundation and can be found at: https://clear.perinatalquality.org/.
Finally, for twin and triplet pregnancies, a number of studies show no benefit for progesterone administration in the reduction of preterm labor. 
Summary: Critical Cervical Length for Treatment with Progesterone
Given a history of previous spontaneous preterm birth at 16 to 37 weeks, irrespective of cervical length, 17-alpha-hydroxyprogesterone caproate 250 mg. is given intramuscularly at 16 to 20 weeks and continued until 36 weeks.  If selective or universal screening is undertaken at 18 through 24 weeks and the transvaginal ultrasound cervical length is ≤ 20 mm., vaginal progesterone is initiated daily with 200 mg. capsules or suppositories or 90 mg. vaginal gel until 36 weeks. 
Tsoi and colleagues found that the transvaginal cervical length (CL) at presentation in singleton women with intact membranes and threatened premature labor predicted delivery within 48 hours, within 7 days, and delivery at < 35 weeks. 
Above. When the CL was ≥ 15 mm no women delivered within 48 hours and that only 0.7% delivered within 7 days. These authors suggest that routine hospitalization may not be necessary in those with threatened preterm labor in the presence of a CL of ≥ 15 mm. Others also report a CL of ≥ 15 mm as the most relevant threshold for predicting delivery in women with threatened preterm labor. 
While CL is an independent predictor of preterm delivery in women with preterm labor, its predictive accuracy as a single measure is considered relatively limited by some.  In addition, the predictive accuracy for CL measurements among women with threatened PTL varies with gestational age at presentation with lower CL cutoff measurements noted at lower gestational ages , while fetal fibronectin (fFn) testing may play a discriminating role in predicting PTL in these women.
Above. In women destined to deliver within 7 days, CL measurements, combined with fetal fibronectin testing improves identification of these women when the CL is between 15 and 30 mm. 
Above. The delivery risk between 7 and 14 days in women with threatened premature labor can be assessed by transvaginal CL and fFn performed on day 7 of women hospitalized for preterm birth.  The delivery risk between 7 and 14 days was increased in the presence of a CL of < 15 mm or a CL of ≥15 to 30 mm and a positive fFn, compared with women with a CL of ≥30 mm.
Preterm Labor and the Risk for Preterm Delivery
Above. After preterm labor is arrested, women are at increased risk for preterm delivery if the cervix is ≥ 1 cm. dilated by digital exam, < 15 mm CL on transvaginal ultrasound and/or a positive fFn. 
Preterm Delivery Calculator
Above. Statistical prediction models have been constructed for individualized assessment of preterm delivery risk within 48 hours and before completed 32 weeks of gestation and an online risk calculator has been constructed.  Data for the calculator includes: fetal fibronectin, transvaginal cervical length in mm and C-reactive protein. A disclaimer is provided and this calculator can be accessed by the following link: http://cemsiis.meduniwien.ac.at/en/kb/science-research/software/clinical-software/prematurebirth/
Low Risk for Preterm Delivery
Above. Women with a transvaginal cervical length of 3.0 cm or greater are unlikely to deliver prematurely despite admission for threatened premature labor. Over 1/3 of women with CL of >3.0 cm are admitted unnecessarily to some hospital units with preterm labor. Yet, few or none deliver prematurely leading to inconvenience and cost , while the addition of fetal fibronectin testing in these women may be cost effective. 
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 dilation 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.
In a review of 5 randomized controlled trials, there is no evidence that cerclage is effective in multiple gestations in preventing preterm births, or in reducing perinatal deaths or neonatal morbidity. 
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. 
Transvaginal ultrasound of 1571 singleton low-risk pregnancies at 37 weeks predicted the onset and outcome of labor.  Women with a cervical length of 10 mm delivered at a mean gestation of 38 weeks, while those with a cervical length of 35 mm delivered at 41 weeks. The following charts from the study’s abstract summarize the findings.
Above. Longer transvaginal cervical lengths measured at 37 weeks are associated with a higher incidence of delivery after 40 weeks 10 days.
Above. Longer transvaginal cervical lengths measured at 37 weeks are associated with a higher rate of C-sections for failure to progress.
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 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 post-dates pregnancies at 41 3/7 weeks to 42 1/7 weeks, among women undergoing induction of labor, 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 (IOL) at 41 3/7ths to 42 1/7ths weeks and per cent of those delivering within 24 hours according to parity. 
Cervical Length and Polyhydramnios
Above. There is a gradual shortening of cervical length in women with poltyhydramnios and a cutoff of 15 mm was associated with a lower gestational age at delivery. 
How to Measure the Cervical Length
Summary: Measurement Technique
Above left. Note the appropriate anatomic landmarks for proper transvaginal ultrasound orientation. Above right. Note the sequential steps for ultrasound performance. A review of image acquisition and technical performance for transvaginal ultrasound is also provided by these authors. 
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 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 external os, follow the posterior cervical lip until the closing point is identified. For the internal os, a small triangle is often seen, place the cursor at the apex of the triangle (the closing point).
Sources of Error
1. Visualize the entire cervix
Above. The entire cervix is not visualized in this example and the internal and external os is not well defined. Although the cervical length is probably normal, this is a suboptimal image.
2. Accurate cursor placement
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.
5. Spontaneous cervical changes
Above. Progressive cervical shortening may occur after transabdominal pressure in patients at risk for cervical incompetence. 
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 to include incorrect interpretation due to vaginal probe orientation and due to cervical distortion by the probe.
Above. Transvaginal cervical length measurement training. Those with no prior ultrasound training can adequately perform the exam after 18 consecutive ultrasound examinations, while those with experience in transvaginal ultrasound required only 1 practice session to learn the technique. 
*National Vital Statistics Report, Volume 64, Number 6, June 17, 2015. (US Department of Health and Human Services)
Above. This image represents a normal transvaginal cervical length with a proper sagittal view, 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 os cervical dilation is 2.5 cm and the approximate external os is 5.2 cm dilatation.
Above. Beaking of the internal os and demonstration of amnion and chorion.
Above. U-shaped funnel measuring 20 mm and cervical length measuring 21 mm.
Above. V-shaped funnel with short cervix. The distal cervix is poor 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 us a post-transabdominal cerclage image. 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.
Transvaginal Ultrasound Videos
Above. Video demonstrates a shortening and funneling of the cervix.
Above. A cerclage suture is in place and the cervix funnels to the still intact suture.
Above. In this example, the amniotic membranes remain intact but are now filling the upper vagina.
Above. Extended V-shaped funnel resulting in a short cervix.
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