Chapter 5. Maternal/fetal Structures: Cervix

Welcome to the chapter on the cervix. This chapter will outline the significance of cervical measurement in predicting preterm labor. It will also detail the proper techniques for measuring cervical length and provide guidance on interpreting these measurements for clinical management.

OBJECTIVES:

By the completion of this Section the participant will be able to:

1. Demonstrate the correct technique for measuring a cervix

2. Describe rationale for using transvaginal vs transperineal approach

3. Cite the normal cervical measurement parameters for less than 22 weeks, and 22-32 weeks

4. Describe degrees of cervical funneling and implications for preterm labor

Reasons for performing cervical length with ultrasound.

Cervical effacement has traditionally been assessed by digital examination, but this method can be unreliable and may not reflect the true cervical length. It had been previously thought that the cervix thins and dilates from outer to inner. However, the opposite occurs in preterm labor; the cervix thins and dilates from the inside out, rendering a digital exam to be inadequate. In cases of premature rupture of membranes or active contractions, digital exams may not be advisable. Ultrasound provides a more accurate assessment.

Measuring the cervix helps assess preterm labor risk. Women with a history of preterm birth are up to twice as likely to experience it again. Current guidelines recommend cervical measurement at around 15-16 weeks for those with a history of preterm labor. There are currently no recommendations to measure the cervix in women in early pregnancy without preterm birth risk factors.

Figure 1. Reasons for performing cervical length measurement with ultrasound.

Berghella’s Cochrane review¹ shows cervical length measurement effectively predicts and helps prevent preterm birth in both symptomatic and asymptomatic women. It can also confirm or rule out incompetent cervix, assess the likelihood of imminent delivery in women with labor symptoms, and, according to ACOG (American College of Obstetricians and Gynecologists)², identify those who may not require intervention.

Figure 2. Reasons for performing cervical length; Berghella et al.

Measuring the cervix may also help:

· Predict success of labor induction (part of the Bishop score)

· Assess risk of pregnancy extending past term

· Guide timing for maternal transport in complication referrals

Cervical length measurement should generally NOT be performed prior to 14 weeks of gestation, as the isthmus and cervix are not distinctly differentiated before this period. For women at risk of preterm labor, cervical screening is recommended between 16 and 24 weeks. Screening beyond 24 weeks in asymptomatic patients has limited clinical utility and does not appear to improve patient outcomes. It is considered an optional procedure by SMFM and ACOG^2,3 ; universal transvaginal cervical length screening in low risk patients should not be mandated.

Figure 3. When to measure cervical length.

Cervical Length Measurement Guidelines

When measuring cervical length, it is advised to take 3 measurements and record the shortest value. This practice accounts for cases where the cervix is considered dynamic, meaning its length may vary during an examination due to internal pressure or contractions.

Lengths to remember are:

4.0 cm is the average length prior to 22 wks.

3.5 cm is the average length from 22-32 wks.⁴

After 32 wks., cervical length is variable, and is not necessarily predictive of labor. Also, after 32 weeks, management would not include tocolysis or trying to stop labor.

A critical measurement to know is 2.5 cm – which represents the 10^th percentile of cervical length. Anything below this value constitutes cervical shortening.

If cervical shortening is detected, consultation or referral for further assessment is imperative. Prior recommendations included use of supplemental progesterone with a cervical measurement of 1.0 to 2.0 cm. The next slide presents the ACOG clinical management pathway for a short cervix.²

Figure 4. Guidelines for measuring cervical length.

Cervical length screening recommendations

Evidence shows cervical length screening is not useful for predicting preterm labor in multiple gestation, PPROM, or placenta previa. Although CL measurement in twin gestation does not reduce the risk of preterm birth, it does have advantages. It can improve timing of antenatal corticosteroid treatment for those who will deliver preterm. While following good measurement principles is important, specific certification in cervical length imaging is not required.

Figure 5. Cervical length screening guidlines, SMFM.

CLEAR certification⁴

CLEAR certification is a U.S. nationally recognized program that provides specialized training in performing accurate cervical measurements. CLEAR stands for: Cervical Length Education and Review program (https://clear.perinatalquality.org). A key requirement for achieving CLEAR certification is the submission of a specified number of images for expert review, which determines eligibility for the certificate of competence. Notably, the imaging criteria required by CLEAR align with the standards presented in this section. Although CLEAR certification is desirable, providers of POCUS (especially those without access to CLEAR certification) can learn to measure a cervix by using the techniques recommended by CLEAR. Other programs offering cervical length training include Fetal Medicine Foundation’s Certificate of Competence in Cervical Assessment (https://fetalmedicine.org)

Figure 6. Cervical length scanning procedure.

CLEAR technique guidelines:

1. Empty maternal bladder

2. Transvaginal ultrasound

3. Avoid undue pressure on the cervix (anterior width should equal posterior width)

4. Cervix should occupy > 75^% of the image

5. Measure the closed portion of the cervix (functional length)

6. Perform 3 measurements over more than 3 minutes

7. Record the shortest length that meets criteria (not the average length)

To perform cervical length measurement, follow these steps⁵:

· Instruct the patient to empty her bladder prior to the examination. Elevating the buttocks with a wedge or pillow on the exam table facilitates posterior wand angulation. An exam table with stirrups may further aid positioning.

· Insert the transvaginal (TV) probe while continuously visualizing the cervix in real-time.

· Once an adequate view is obtained, withdraw the probe slightly to visualize the full thickness of the cervix.

· Ensure that the entire length of the cervical canal, including both the internal and external os, is clearly observed.

· The measurement can be conducted via either the transvaginal or transperineal (TPUS) approach.

· Utilizing a higher frequency setting—such as in TVUS—is recommended for optimal visualization of os details.

· When comparing TVUS and TPUS, studies show that in 95% of cases, the difference between measurements is within ±5 mm.*

· Although the transperineal/translabial approach is less invasive, mastering probe orientation may present additional challenges.

*Based on data comparing the two imaging modalities.

How to measure the cervix

Once the cervix is visible in the long plane, zoom in so it fills 50-75% of the image for clear morphology. Identify the cervical mucosa, which may appear hyperechoic or hypoechoic.

Figure 7., Images 1, 2. How to measure the cervix, with image examples.

Figure 8, Images 3, 4. How to measure the cervix, keeping anterior and posterior cervical lips equal.

To identify the internal os, find a small dip or wing-shaped indentation. The external os is typically just inside the outer edge of the cervix, not at its very tip.

Figure 9, Image 5. Illustration and image demonstrating identification of internal and external cervical os.

When the cervix is visible in the long plane, increasing the zoom or magnification helps to clearly identify its morphology.

Measuring a curved cervix

The images below (image 6 and image 7) show two methods for measuring a curved cervix: measure each segment separately and then add the 2 measurements, or estimate using a straight line.

Image 6. How to measure a curved cervix with straight lines.

Using the “trace” function on the machine allows measurement with a curved line.

Image 7. How to measure a curved cervix with tracing.

Cervical elongation

Measuring a cervix with a full bladder can result in elongation or stretching due to bladder pressure. Typically, a normal cervix measures no more than 4 cm.

Image 8. Image of cervical elongation due to a full bladder, transabdominal view.

The images below (images 9, 10, and 11) show different transabdominal views of the cervix. If a transvaginal probe is unavailable, an estimate of cervical length can be obtained; however, this measurement is less accurate than with a transvaginal scan. A full bladder improves visualization of the cervix due to acoustic enhancement, but it may also falsely increase the apparent cervical length, which can result in a misleading measurement.

Images 9, 10, 11. Images of cervical measurements as examples.

Images 12 and 13 below demonstrate how cervix measurement may vary depending on whether the bladder is full or partially empty during a transabdominal assessment.

Images 12, 13. Images comparing cervical length with full versus empty bladder.

The transabdominal image below (imafe 14) shows a cervical measurement at term. The cervix exhibits a hyperechoic os, likely indicating the presence of a mucus plug. It is important to note that bladder distension may be contributing to an elongated appearance of the cervix on this examination.

Image 14. Cervical measurement at term, transabdominal view.

Clinical management

Chart 1 below outlines treatment for a short cervix. Progesterone treatment is still recommended for those with a short cervical length (CL), but not to be used prophylactically or those with a normal CL, even for those with a history of preterm birth.

Chart 1. Algorithm for management of a short cervix, ACOG.

Funneling

Another important parameter when assessing the cervix is funneling, which refers to the protrusion of the amniotic membranes into the cervical canal and progressive dilation originating at the internal os. While some consider funneling an additional risk factor for preterm delivery, recent studies indicate that cervical length (functional length) and whether the cervix remains closed are more significant predictors.

Figure 10. Description of cervical funneling.

In years past, sonographers were taught to measure the funnel width and length to indicate the degree of funneling. Now, however, focus is placed on the functional cervical length (the part of the cervix that remains closed).

Figure 11. Illustration of prior method of measuring cervical funneling.

Funneling develops in stages as pressure increases on the cervix. A T shape means the internal os remains closed, indicating no funneling at that site.

Figure 12. Illustration of the “T-shape” representing absence of cervical funneling.

Image 15 (below) demonstrates that the cervix has no funneling – and has a “T-like” shape at the internal os

Image 15. Image of cervix with a “T-shape”, or absence of funneling.

Image 16. Image of cervical measurement deomonstrating absence of funneling.

In image 16, although we see some fluid in front of the fetal head, this is not funneling, since the internal os is contiguous with the lower uterine segment and is not open.

Funneling - the “Y” shape

A Y-shaped opening of the internal os occurs when there is increased pressure applied to the cervix.

Figure 13. Illustration of the “Y-shape” of early cervical funneling.

Image 17 displays the “Y-shape”.

Image 17. Image of a cervix showing the “Y-shape” of early cervical funneling.

V shape

The next stage is called the V-shape, where the internal os is open most of the way to the external os, leaving very little intact cervix

Figure 14. Illustration of the “V-shape” of progressive cervical funneling.

U -shaped funneling

The next step in the progression is the “u” shape. This is where the external os is stretched as well.

Figure 15. Illustration of the “U-shape’ of significant cervical funneling.

Below is an example of U-shaped funneling (image 18), with only a tiny part of the external os still intact.

Distinguishing true cervical funneling versus distortion of the image from a uterine contraction can be challenging. Here, the cervix appears closed and long; extending observation time may help clarify if funneling is present.

Image 18. Image of a cervix with “U-shape” funneling and only a tiny measurable cervical os.

Image 19 shows a transabdominal view of a cervix with a full bladder. Although one might look at this at first and think it is a U-shaped funneling, look at the functional length of the cervix – it appears intact all the way up to the internal os. This is just a pocket of fluid that has collected in front of the fetal head.

Image 19. Image of a cervix with what initially appears as funneling; on closer inspection the bulge was caused by a cervical contraction distorting the lower uterine segment, while the functional cervical length is appropriate.

Image 20 shows that the bladder is elongating the cervix, so the cervix may actually be shorter than it appears.

Image 20. Image of what first appears as cervical funneling, but the functional cervical length is good (4.09 cm). Keeping in mind that the maternal bladder is full and elongating the cervix, it misrepresents the true funcional cervical length. Also, there is an amniotic fluid pocket between the fetal head and the cervix.

The images below (images 21 and 22) display short cervices: 1.9 cm on the left and 1.0 cm on the right. The left image shows a near-term fetus, making a short cervix less significant, while the right image depicts a second trimester fetus, where a short cervix is more clinically relevant.

Image 21, 22. Images of short cervix.

Fluid sludge at internal os

Amniotic fluid sludge is identified as an irregular, mixed, or hyperechoic area near the internal os during cervical examination. This observation has been associated with infection and an elevated risk of spontaneous preterm delivery, premature membrane rupture, and chorioamnionitis. Detection of amniotic fluid sludge typically leads to consideration of consultation or referral.

Figure 16, Image 23. Description and image of amniotic sludge near cervical os.

SUMMARY

The rationale for measuring the cervix is to determine abnormal shortening to detect the potential of preterm labor in women who are at risk.

Cervical length can be initiated as soon as the cervix becomes differentiated from the uterus (not before 14 weeks’ gestation).

Screening of high-risk women should occur between 16 and 24 weeks.

Screening after 24 weeks in asymptomatic women provides limited clinical value and does not appear to improve outcomes.

The cervix is ideally measured with an empty bladder, using either a transvaginal or transperineal approach.

Normal cervical lengths are:

< 22 weeks: 40 mm

22-32 weeks: 35 mm (10^th percentile is 25mm or 2.5 cm)

Cervical funneling can imply risk of preterm labor.

The practitioner must consult or refer for higher level management if a cervix is deemed to be shortening, especially in early second trimester.

It is noteworthy that short cervical length is not always associated with preterm labor or birth; the integrity of the functional length determines whether there will be early dilation.⁴

Women's Point-of-Care Ultrasound: Second and Third Trimester

Maternal Fetal Structures Cervix