Transvaginal sonography is a safe and acceptable method of studying the cervix, as it is well accepted by >99% of women, and pain is reportedly felt in <2% of the cases.¹⁰ A report by Carlan et al¹¹ showed that TVU of the cervix does not result in inoculation of bacteria or increased risk of infection for the mother or fetus compared with women who do not undergo transvaginal sonography.

Transvaginal cervical sonography is unlike any other method used to evaluate the cervix because it can accurately assess cervical insufficiency at an early asymptomatic stage, at a point where preventative measures may be used to thwart PTB. Cervical changes visible on TVU include the initial opening of the internal cervical os, progressive cervical widening and shortening of the endocervical canal from internal to the external os, and dilation of the external os.³

Optimal Timing and Frequency for Measuring Cervical Length Via Transvaginal Ultrasound

Prior to 14 weeks gestation, almost all women, including those at the highest risk of PTB, will have a normal CL. CL of <25 mm at this gestation is seen only in women who have had a previous second-trimester loss or those with a history of a large cervical cone biopsy.¹² A CL of 25-50 mm is normal at 14-24 weeks in all pregnant women (Fig. 1). In low-risk women, mean CL at 14-30 weeks of gestation is 35-40 mm, with the lower 10th percentile being 25 mm and the upper 10th percentile being 50 mm.¹³ Optimal timing for measuring CL must be established because if attempted too early, the lower uterine segment may be too difficult to separate from the true cervix. In addition, after 30 weeks, the cervix normally shortens progressively in preparation for term labor, so a CL < 25 mm after 30 weeks is physiological and not indicative of PTB in asymptomatic women.⁷

In most women who will have a PTB, a short CL is first noted at approximately 18-22 weeks of gestation; therefore, initial screening should be started at this time.¹³ The earlier the short CL is detected, the higher the likelihood of PTB. There also exists an inverse relationship between CL and PTL, in that the shorter the cervix, the greater the likelihood that PTB will occur. In the highest risk women, a CL < 25 mm has a positive predictive value of 70% for PTB at <35 weeks when detected at 14-18 weeks, and of 40% when detected at 18-22 weeks.⁷ This observation indicates that it may be beneficial to screen these high-risk women earlier than 18 weeks to determine their need for intervention (Fig. 3).

The benefit of repeated transvaginal sonographic examinations and the ideal interval for repeating scans have not been clearly established in the published reports. Given the variation in CL based on risk of PTB, current research shows that it may be best to break down transvaginal ultrasonographic screening into 3 groups: those at low risk, high risk, and very high risk for PTB. If a screening program were employed in relatively low-risk women, 1 TVU at approximately 18-22 weeks would probably be sufficient, with no need to repeat the scan in the future.³ In high-risk women, it seems that 2 normal CLs, 1 at 14-18 weeks of gestation and another at 18-22 weeks is enough. Finally, in very high-risk women, including those with a previous second-trimester loss or very early spontaneous PTB, it is best to undergo serial transvaginal examinations every 2 weeks from 14 to 24 weeks of gestation.^{[3] and [14]} Appropriate timing of CL measurement is necessary so that changes leading to PTB are detected early enough in the process to allow for intervention.

Cervical Evaluation: What to Measure and What Not to Measure

Cervical Length

Although multiple cervical parameters have been evaluated as predictors of PTB, CL continues to be the most reproducible and reliable indicator. Accurate measurements of CL begin at the internal os, follow the path along the endocervical canal, and end at the external os. If the cervical canal is curved (a deviation of the canal >5 mm from a straight line from the internal to external os), then the canal can either be traced, or the sum of 2 straight lines that follow the curve of the canal can be used.¹⁵ If the endocervical canal takes on a curvilinear appearance, this is a reassuring finding because a curved cervix usually signifies a CL >25 mm (Fig. 1). A worrisome finding would be a short, straight endocervical canal.

Funneling

Funneling of the cervix is defined as the opening of the internal cervical os on ultrasound. In about 10% of low-risk women¹² and 25%-33% of high-risk women,^{[7] and [15]} the internal os is open in the second trimester (Figure 2 and Figure 3). The open portion of the cervix is the funnel length and the internal diameter is the funnel width. Percent funneling is defined as funnel length divided by total CL, in which total CL is equal to the sum of funnel length and functional length. Functional CL is defined as the portion of endocervical canal that remains closed. Functional CL is the measurement that is typically used for calculations and predictions of PTB (Figure 2 and Figure 3).

Funneling of the internal portion of the cervix occurs along a continuum. A normal closed cervix takes on a T appearance. As the cervix begins to show funneling, the first shape it morphs into is that of a Y. A Y shape represents a small funnel, which if it includes <25% of the cervix, it is not a clinically significant finding.¹⁶ Next, the cervical funneling takes the shape of a V. This represents a more significant funnel that extends closer to the external cervical os. Finally, the most ominous funnel shape is that of a U as this shape is the most indicative of PTB.¹⁷ Dr Iams has created the following pneumonic for the progression TYVU: “Trust Your Vaginal Ultrasound”!

Evaluation of cervical funneling should take place over the course of at least 5 minutes to resolve any question of morphology of the upper cervical canal. Not only may uterine segment contractions mimic the appearance of funneling, but the funneled portion of the cervix may also merge with the lower uterine segment and distort the image. There is higher interobserver variability among trained sonographers when detecting cervical funneling as opposed to when studying CL.¹²

Despite the high interobserver variability, funneling has been shown to have good predictive accuracy for PTB. In a study of high-risk women, minimal funneling (<25%) between 14 and 22 weeks was not associated with a significant increased risk of PTB. In contrast, moderate (25%-50%) and severe (>50%) funneling were both associated with a ≥ 50% likelihood of PTB.⁷ If funneling is present, the CL is typically <25 mm.¹² Compared with a CL of <25 mm alone, CL plus the presence of funneling will increase the sensitivity of predicting PTB from 61% to 74%, without changing specificity and positive and negative predictive values.¹⁴ In addition, the risk of PTB has been found to be higher in instances in which both a short CL (<25 mm) and funneling is detected, as opposed to short CL alone.¹⁸ In contrast, if a normal CL of ≥25 mm is present; the additional finding of funneling does not increase the risk of PTB.¹⁹ In general, it is most important to report CL, and we prefer, in most cases, not to report percent funneling even if present, because it does not affect clinical management. Interventions based on TVU have been based on a short CL, and not solely on the presence of funneling.

Sludge

Intra-amniotic sludge on ultrasonography appears as a cluster of free-floating hyperechogenic material within the amniotic fluid near the uterine cervix. Sludge is an independent risk factor for histologic chorioamnionitis and microbial invasion of the amniotic cavity in women with spontaneous PTL and intact membranes.²⁰ Moreover, the presence of sludge is an independent risk factor for preterm premature rupture of membranes and spontaneous preterm delivery. Espinoza et al²¹ in a study showed that 71% of women who were found to have intra-amniotic sludge went into PTL within 7 days, compared to 16% of women without the presence of sludge. The combination of “sludge” and a short cervix (<25 mm) confers a higher risk for spontaneous preterm delivery at <28 and <32 weeks than that of a short cervix alone.²¹ However, although intra-amniotic sludge has been shown to be predictive of PTB, there is insufficient evidence to assess whether measuring the presence of sludge alone improves the predictive accuracy that is already provided by CL.

Three-Dimensional Ultrasound

Three-dimensional ultrasound can be used to assess CL. The true CL is more easily identified with all 3 planes available. At times, funneling is only detectable on planes other that the 2-dimensional sagittal plane. Despite these advantages, 3-dimensional ultrasound is not necessary in clinical practice to assess CL.³

Other Measurements

Many other parameters have been studied in transvaginal sonographic imaging as factors to predict PTB. Examples include funnel width, funnel length, endocervical canal dilatation, anterior and posterior cervical width, cervical position (horizontal vs vertical), lower uterine segment thickness, cervical angle, visibility of chorion at internal os, cervical index (funnel length + 1/functional length), and vascularity. Among those mentioned, none have proven to be more reliable or predictive of PTB than CL.^{[3] and [22]}

Factors Affecting Prediction

TVU CL has been shown to be predictive in all populations studied (Table 3).^{[12], [15], [23], [24], [25], [26], [27] and [28]} One of the most important features of this screening test for PTB is its sensitivity. Sensitivity represents the detection rate—the percent of women who will deliver preterm that are detected during asymptomatic weeks before the PTB by visualizing a short CL on TVU in the second trimester. This prediction is highly dependent on several factors:

1 Number of Fetuses. TVU CL is most effective as a predictor of PTB in singleton gestations. In multiple gestations, most of the women who eventually deliver preterm do not manifest a short CL in the second trimester. The sensitivity of this test is <50% in multiple gestations (Table 3).^{[12], [15], [23], [24], [25], [26], [27] and [28]}

 

2 Ob-Gyn Risk Factors. TVU CL is most sensitive in singleton gestations with poor obstetrical history. The population most studied has been singleton gestations with previous PTB. In these women, screening with TVU CL in the second trimester is associated with high sensitivity, as over two-thirds of women destined to deliver preterm can be detected early while asymptomatic by this test¹⁵ (Table 3; Fig. 3). Sensitivity remains >50% in women with singleton gestations and other risk factors for PTB, such as previous cone biopsy, Mullerian anomaly, or previous multiple dilation and evacuations (D&Es) (Table 3).^{[23], [24] and [25]} The sensitivity is also high in singleton gestations with PTL (Table 3).²⁸ It is not surprising that interventions aimed at preventing PTB based on TVU CL screening seems to be most effective in these populations of singleton gestations with risk factors for PTB. On the contrary, as the sensitivity is ≤30% in multiple gestations, TVU CL is an ineffective screening test for PTB in these populations.^{[26] and [27]}

In women with singleton gestations, but no particular risk factors for PTB, the sensitivity of this test is only 37% (Table 3).¹² Therefore, most of these women, who represent the biggest group of women delivering preterm, do not seem to develop TVU CL shortening in the second trimester. This may be the possible reason for the ineffectiveness of intervention based on TVU CL screening so far in this population.

3 CL. The shorter is the CL, the higher is the risk of delivering preterm (Table 4).²⁹ Therefore, CL and predicted gestational age at delivery are directly proportional. A CL of 25 mm represents the 10th and 25th percentiles of distribution of CL for the general (low-risk) and “prior PTB” population of singleton gestations, respectively. For ease of clinical use, 25 mm has been chosen as the cut-off at and above which a cervix can be called “normal,” and below which it can be called “short.” A CL of <25 mm at or before 28 weeks is always abnormal and associated with a higher incidence of PTB (Table 4).²⁹

Causes of Short Cervical Length

It is unclear what exactly “causes” PTB, and so it is unclear what “causes” a shortening of CL in the second trimester. As we have seen, contractions can shorten the cervix. The contrary is probably also true, that is, a short CL can eventually be associated with contractions. The 2 probably have a common etiology in most cases. A short CL is associated with intrauterine infection. Once again, it is unclear whether first bacteria “weaken” and shorten the cervix, or it is cervical shortening that allows the vaginal flora to invade and infect the originally sterile uterus. Both these mechanisms can occur, and create a “catch 22” phenomenon.

Women with multiple D&Es often develop a short cervix in the second trimester. To us, these are the women in whom a cervical-insufficiency mechanism causing the short cervix is most evident. On the contrary, in twins the cervix shortens only at the very end, just before clinical signs, and not because of an abnormal cervix, but because of excessive pressure from the quickly enlarging uterus above.

We encourage further research in the many yet unknown mechanisms responsible for the development of a short CL in the second trimester.

Conclusions

CL measurement by TVU in the second trimester is one of the most effective screening methods for the prediction of PTB. To achieve adequate prediction, proper technique of TVU CL measurement should be followed (Table 1),³ with particular precautions (Table 2). CL is the most predictive of the findings on the cervix at TVU. Prediction of PTB varies widely depending on several factors, particularly the population studied (Table 3).^{[12], [15], [23], [24], [25], [26], [27] and [28]} Its sensitivity is highest in asymptomatic singleton gestations with risk factors for PTB (previous PTB, cone biopsy, Mullerian anomalies, and greater than or equal to D&Es), and in symptomatic singleton gestations with PTL. In all populations, the shorter is the CL, and the earlier it is detected in pregnancy, the higher is the incidence of PTB (Table 4).²⁹