Short cervix before 24 weeks: Screening and management in singleton pregnancies

INTRODUCTION — Prelabor preterm cervical shortening, particularly before 24 weeks of gestation, is associated with an increased risk for spontaneous preterm birth, which is a major cause of infant morbidity and mortality. Detection of a short cervix in the second trimester is useful because providing progesterone supplementation or performing a cerclage may prolong the gestation compared with expectant management.

This topic will review issues related to sonographic cervical length screening before 24 weeks for prediction of spontaneous preterm birth in patients with singleton pregnancies and management of those found to have a short cervix. The utility of measurement of cervical length in twin pregnancies and in the evaluation of suspected preterm labor is reviewed separately. (See "Twin pregnancy: Routine prenatal care", section on 'Screening for short cervical length' and "Preterm labor: Clinical findings, diagnostic evaluation, and initial treatment", section on 'Transvaginal ultrasound examination'.)

RATIONALE FOR MEASURING CERVICAL LENGTH — Cervical shortening is one of the first steps in the processes leading to labor and can precede labor by several weeks. The cause is often unclear. It has been attributed to occult uterine activity, uterine overdistention, congenital or acquired cervical insufficiency, decidual hemorrhage, infection, inflammation, and biological variation.

A short cervical length before 24 weeks is predictive of subsequent spontaneous preterm birth and is most predictive in patients with early and substantial cervical shortening [1-6] and in those with a history of early and/or repeated spontaneous preterm births [7]. Because effacement begins at the internal cervical os and progresses caudally [1,5], a short cervix is often detected on ultrasound examination before it can be appreciated on physical examination.

In patients with a previous spontaneous preterm birth, identification of a short cervix suggests cervical insufficiency. Placement of a cerclage (termed ultrasound-indicated cerclage) in this subgroup of patients with a short cervix can prolong the gestation and improve pregnancy outcome. These data are also reviewed separately. (See "Cervical insufficiency", section on 'Ultrasound-based diagnosis' and "Cervical insufficiency", section on 'Ultrasound-based cervical insufficiency'.)

In patients without a previous spontaneous preterm birth, identification of a short cervix suggests an increased risk for spontaneous preterm birth in the current pregnancy; treatment with vaginal progesterone can reduce this risk and its sequelae. The evidence for and management of progesterone supplementation to reduce the risk of preterm birth are discussed in detail separately. (See "Progesterone supplementation to reduce the risk of spontaneous preterm labor and birth", section on 'Candidates for progesterone supplementation'.)

DIAGNOSIS OF SHORT CERVIX — Before 24 weeks, we make the diagnosis of a short cervix when transvaginal ultrasound cervical length is ≤25 mm (ie, 2^nd to 3^rd centile), regardless of the patient’s obstetric history (no prior preterm birth, prior preterm birth, singleton pregnancy, twin pregnancy). We consider this an appropriate diagnostic threshold because meta-analyses of randomized trials of therapeutic interventions (progesterone, cerclage) initiated at this threshold in patients with singleton pregnancies with or without a prior spontaneous preterm birth report a 30 to 40 percent reduction in preterm birth compared with no intervention [8-10].

Worldwide, there is some variation in the cervical length threshold that triggers intervention (progesterone, cerclage) in nonlaboring patients [11]. For example, some clinical guidelines use <15 mm (0.5^th centile) and others use <20 mm (1^st centile). The choice reflects a variety of factors, such as the importance placed on sensitivity versus specificity, whether there is a desire to use the threshold from a specific intervention trial, and the patient population. The American College of Obstetricians and Gynecologists uses <25 mm for patients with singleton pregnancies, regardless of past obstetric history [12]. (See 'Clinical approach' below.)

The reported sensitivity of prelabor cervical length ≤25 mm for preterm birth varies from 6 to 76 percent in the literature [13]. This variation is due in large part to the populations studied and also to methodologic differences among studies. There is, however, general agreement that the overall risk of preterm birth increases as cervical length decreases below 25 mm and the relationship is strongest when a short cervix is observed before 24 weeks of gestation or in patients with a prior spontaneous preterm birth, especially before 32 weeks [1-7,14-16]. By comparison, in the third trimester, the relationship between short cervical length and preterm birth is weak and preterm birth within two weeks of the diagnosis is highly unlikely regardless of the cervical length [17].

There is no threshold value below which the patient always delivers remote from term. In one study of patients with no measurable cervical length at 14 to 28 weeks, 25 percent gave birth at ≥32 weeks [18]. In another study of patients with cervical length ≤25 mm at 24 weeks, 82 percent gave birth at ≥35 weeks; of those with cervical length ≤13 mm at 24 weeks, 50 percent gave birth at ≥35 weeks [1].

Of note, the diagnosis of short cervix is generally limited to pregnant people. Cervical length measurements performed in nonpregnant females are not useful for predicting spontaneous preterm birth [19].

CLINICAL APPROACH

Overview — We perform cervical length screening with transvaginal ultrasound (TVUS) in all pregnancies by 24 weeks of gestation, regardless of obstetric history or number of fetuses, as summarized in the algorithm (algorithm 1) and discussed in the following sections of this topic. Although there is consensus for cervical length screening in singleton pregnancies less than 24 weeks of gestation at high risk for spontaneous preterm birth (eg, prior spontaneous preterm birth [11]), the value of using TVUS in all pregnancies is controversial [20-23]. Some UpToDate contributors use transabdominal ultrasound for cervical length screening in pregnancies at average risk of preterm birth, repeating the examination with TVUS if the cervix is not well imaged or if it appears to be short (see 'Screening with transvaginal versus a combination of transabdominal and transvaginal ultrasound' below). Positions of some obstetric societies are as follows:

●Society for Maternal-Fetal Medicine (SMFM) – SMFM recommends routine cervical length screening with TVUS between 16 and 24 weeks of gestation for patients with a singleton pregnancy and history of prior spontaneous preterm birth [24]. They consider TVUS cervical length screening reasonable for patients with a singleton pregnancy and no history of prior spontaneous preterm birth but have not recommended routine screening for this population. They recommend not performing routine cervical length screening for patients with a cervical cerclage, preterm prelabor rupture of membranes, or placenta previa.

●American College of Obstetricians and Gynecologists (ACOG) – In a practice bulletin on preterm birth, ACOG concluded that cervical length screening with serial TVUS is indicated for singleton pregnancies when there is history of a prior spontaneous preterm birth [12]. In the absence of a prior spontaneous preterm birth, ACOG recommended imaging the cervix with either a transabdominal or TVUS approach at the 18+0 to 22+6 weeks of gestation anatomy.

●International Federation of Gynecology and Obstetrics (FIGO) – FIGO recommends sonographic cervical length screening in all patients with a singleton pregnancy at 19+0 to 23+6 weeks of gestation using TVUS [25].

The value of universal screening is based on evidence from several studies. For example, in a large observational study, the introduction of universal cervical length screening in singleton gestations without prior spontaneous preterm birth was associated with significant reductions in the frequency of spontaneous preterm birth compared with the period before screening was implemented [26]:

●Births <37 weeks of gestation (4.8 versus 4 percent, adjusted odds ratio [aOR] 0.81, 95% CI 0.75-0.89)

●Births <34 weeks (1.3 versus 1 percent, aOR 0.78, 95% CI 0.66-0.93)

●Births <32 weeks (0.7 versus 0.5 percent, aOR 0.76, 95% CI 0.60-0.95)

Another study found that restricting screening to patients with historical risk factors for preterm birth would miss approximately 40 percent of those with a short cervix and thus at risk for preterm birth [27]. The number needed to screen to prevent one preterm birth was:

●Universal screening – 913 (95% CI 591-1494)

●One risk factor for preterm birth – 474 (95% CI 291-892)

●Two risk factors for preterm birth – 125 (95% CI 56-399)

The study also found an association between a prior indicated preterm birth and a short cervix in a subsequent pregnancy, consistent with other data that patients with a prior indicated preterm birth are at increased risk of subsequent spontaneous preterm birth, presumably as a result of a common pathophysiologic etiology [28-30].

However, the value of universal screening has not been proven. A 2019 meta-analysis of randomized trials did not find sufficient evidence to recommend for or against routine cervical length screening for all pregnant people because of limitations of the included trials [31]. For example, the threshold for short cervix and timing of the screening examination(s) varied among the trials; there was no standard protocol for management of patients based on cervical length, and the populations were heterogeneous. Population heterogeneity is important since population characteristics that could affect the performance of the test include the proportion of singleton versus multiple gestations, symptomatic versus asymptomatic patients, intact membranes versus ruptured membranes, prior spontaneous preterm birth versus no prior spontaneous preterm birth, prior indicated preterm birth versus prior spontaneous preterm birth, prior term birth versus no prior term birth, and prior cervical surgery versus no prior cervical surgery [1,7,32-39].

Specific patient populations

Nulliparous patients and parous patients with no prior spontaneous preterm singleton birth

●Screening protocol — For patients with singleton pregnancies and no history of prior spontaneous preterm birth, we screen for a short cervix with a single TVUS examination at approximately 20 weeks (18 to 24 weeks) (algorithm 1) [12,40].

Approximately 1 percent of patients have a short cervix at the author's institution [41]. The rate is slightly higher in nulliparous patients (1.3 to 5.4 percent in one large study [42]) than in parous patients without a prior spontaneous preterm birth [43]. Because the rates of spontaneous preterm birth <37, <34, and <32 weeks in patients with a short cervix appear to be similar for both groups, we use the same screening protocol for nulliparous patients and parous patients with no prior spontaneous preterm birth.

●Management of patients with a short cervix — We treat patients with singleton gestations, no prior spontaneous preterm births, and a short cervix with vaginal progesterone [12]. The evidence for this approach is reviewed separately. (See "Progesterone supplementation to reduce the risk of spontaneous preterm labor and birth", section on 'Candidates for progesterone supplementation'.)

Other possible interventions:

•Cerclage – We suggest that clinicians discuss the available data and its limitations with the patient and make a shared decision regarding placement of a cerclage. This decision may vary depending on whether the cervical length is ≤10 mm versus ≤15 mm versus ≤20 mm versus ≤25 mm, risk factors for preterm birth, cervical and membrane appearance on speculum examination,  on digital examination, and the patient's values and preferences. Cervical cerclage is not routinely recommended for patients with a short cervix who have not had a prior spontaneous preterm birth since a diagnosis of cervical insufficiency has not been established and many of these patients will have a term or near term birth without surgery. However, available data are limited (discussed below), and practice varies among clinicians.

In a meta-analysis of individual patient data from five randomized trials in which singleton pregnancies without prior spontaneous preterm birth were randomly assigned to cerclage or no cerclage if the cervix was short, cerclage placement did not result in significant reduction in birth <35 weeks (21.9 versus 27.7 percent, relative risk [RR] 0.88, 95% CI 0.63-1.23) [44]. However, planned subgroup analyses suggested efficacy in patients with cervical length <10 mm (preterm birth <35 weeks: 39.5 versus 58 percent, RR 0.68, 95% CI 0.47-0.98). Observational data support this finding: cerclage placement has been associated with superior neonatal outcome compared with progesterone in patients with very short (<8 to 10 mm) cervical lengths [45-47].

•Pessary – Use of a pessary rather than progesterone in patients with a short cervical length has been proposed as an effective, inexpensive, and easy-to-implement method for prolonging pregnancy. Efficacy is not supported by meta-analyses of randomized trials [48,49], although some individual trials have reported a reduction in births <34 weeks of gestation. (See "Cervical insufficiency", section on 'Pessary'.)

•Bed rest – Bed rest is not recommended in patients with a short cervical length. It does not prolong pregnancy, increases the risk for venous thromboembolic events, and may actually increase the risk for preterm birth. These data are reviewed separately. (See "Preterm birth: Risk factors, interventions for risk reduction, and maternal prognosis", section on 'Bed rest is not helpful'.)

Parous patients with a prior spontaneous preterm singleton birth

●Screening protocol — For patients with a singleton pregnancy and a history of prior spontaneous preterm birth, we begin TVUS cervical length screening at 14 to 16 weeks of gestation (the earlier the prior spontaneous preterm birth, the earlier the screening) and perform serial examinations as shown in the algorithm (algorithm 1). Serial screening was more effective than a single screen in large trials of screening in this population [7,50].

●Management of patients with a short cervix — Patients with a prior spontaneous preterm birth are at high risk for recurrence and are offered progesterone supplementation (vaginal or intramuscular) to reduce this risk based on their history of spontaneous preterm birth alone [12]. (See "Preterm birth: Risk factors, interventions for risk reduction, and maternal prognosis", section on 'History of spontaneous preterm birth' and "Progesterone supplementation to reduce the risk of spontaneous preterm labor and birth", section on 'Candidates for progesterone supplementation'.)

If a short cervix is subsequently identified in a patient with a prior spontaneous preterm birth, we make a diagnosis of cervical insufficiency and offer cerclage in addition to continuing progesterone [12]. The rationale/evidence for this approach and management of these pregnancies are discussed in detail separately. (See "Cervical insufficiency", section on 'Ultrasound-based diagnosis' and "Cervical insufficiency", section on 'Ultrasound-based cervical insufficiency'.)

Patients with risk factors for but no prior spontaneous preterm birth

●Screening protocol — For patients with singleton pregnancies with risk factors for spontaneous preterm birth but no prior spontaneous preterm birth, we screen for a short cervix using a single TVUS examination at approximately 20 weeks (18 to 24 weeks). These patients may be nulliparous or parous with new risk factors (eg, conization) that arose after their previous deliveries.

This approach is the same as that for any patient without a previous spontaneous preterm birth. We do not use a different screening protocol for patients with risk factors because their pregnancy outcome needs to be established before committing them to serial cervical length surveillance and possibly a cervical procedure (cerclage) that may be unnecessary. Although a minority of these patients develop cervical insufficiency, most do not; therefore, we believe the pregnancy course and outcome need to be evaluated before making this diagnosis.

●Management of patients with a short cervix — We manage patients with risk factors for but no previous preterm birth who have a short cervix in the same way as described above for patients without a history of preterm birth who develop a short cervix: treat with vaginal progesterone. (See 'Nulliparous patients and parous patients with no prior spontaneous preterm singleton birth' above.)

Parous patients with a prior spontaneous twin birth — The best approach to patients with a prior spontaneous twin birth is controversial. Some studies have reported that a prior spontaneous twin birth is associated with an increased risk of spontaneous preterm birth in the subsequent singleton pregnancy [51-53]. The increased risk appears to be limited to previous twin births <34 weeks [51,53].

●Screening protocol

•Prior twin spontaneous preterm birth ≥34 weeks – We screen for a short cervix with a single TVUS examination at approximately 20 weeks (18 to 24 weeks) as in singleton pregnancies with no prior preterm birth (algorithm 1).

•Prior twin spontaneous preterm birth <34 weeks – We begin TVUS cervical length screening at 14 to 16 weeks of gestation (the earlier the prior spontaneous preterm birth, the earlier the screening) and perform serial examinations as in singleton pregnancies with a prior preterm birth (algorithm 1).

●Management of patients with a short cervix

•Prior twin spontaneous preterm birth ≥34 weeks – For patients with a past history of late preterm spontaneous twin birth who develop a short cervix in the subsequent singleton pregnancy, we offer vaginal progesterone supplementation upon diagnosis of a short cervix. Management is similar to that in patients with singleton gestations, no prior spontaneous preterm births, and a short cervix. (See 'Nulliparous patients and parous patients with no prior spontaneous preterm singleton birth' above.)

•Prior twin spontaneous preterm birth <34 weeks – Patients with a prior early spontaneous preterm birth are at high risk for recurrence and are offered progesterone supplementation to reduce this risk. (See "Preterm birth: Risk factors, interventions for risk reduction, and maternal prognosis", section on 'History of spontaneous preterm birth'.)

If a short cervix subsequently develops, we make a diagnosis of cervical insufficiency and offer cerclage in addition to progesterone. The rationale/evidence for this approach and management of these pregnancies are discussed in detail separately. (See "Cervical insufficiency", section on 'Ultrasound-based cervical insufficiency'.)

Patients with a multiple gestation — The screening protocol for patients with twins and management of patients with a short cervix is reviewed separately. (See "Twin pregnancy: Management of pregnancy complications", section on 'Approach to patients with a short cervix'.)

PROCEDURE FOR SONOGRAPHIC MEASUREMENT OF CERVICAL LENGTH

Basis for timing the first and last screening test — Cervical length is affected by gestational age but not significantly affected by parity, race/ethnicity, or maternal height [1-3,54-58].

Reproducible measurement of cervical length usually becomes possible at approximately 14 weeks of gestation and is consistently possible by 16 to 18 weeks when the cervix normally becomes distinct from the lower uterine segment [40]. Cervical length measurements before 14 weeks of gestation have limited clinical value [40,59]. However, in some particularly high-risk pregnancies, such as those with prior second-trimester losses and/or large (or multiple) excisional biopsies, cervical shortening has been seen as early as 10 to 13 weeks of gestation and was associated with a high risk of second-trimester loss [40].

Normally, cervical length is stable between 14 and 28 weeks of gestation and is described by a bell-shaped curve [1,60]. Approximately 90 percent of nulliparous patients with singleton pregnancies have cervical length >30 mm between 16 and 22 weeks of gestation [42]. The median cervical length is 40 mm before 22 weeks, 35 mm at 22 to 32 weeks, and 30 mm after 32 weeks.

Screening is discontinued at 24 weeks because intervention trials have begun treatment by 24 weeks of gestation. After 30 weeks, cervical length measurement is generally not useful for predicting spontaneous preterm birth because, as noted above, the cervix physiologically starts to shorten at this time, even in patients destined to deliver at term.

Screening with transvaginal versus a combination of transabdominal and transvaginal ultrasound — We perform transvaginal ultrasound (TVUS) cervical length screening in all pregnancies because TVUS cervical measurements are more reproducible and reliable than those obtained by transabdominal ultrasound (TAUS) and more sensitive for prediction of spontaneous preterm birth [61-68]. It is also important to note that all randomized trials supporting the efficacy of treatment of patients with a short cervix used TVUS to measure cervical length [32,50,60,69-73].

Another approach used by some clinicians is to measure cervical length by TVUS routinely in patients with risk factors for spontaneous preterm birth. However, in patients thought to be at low risk for spontaneous preterm birth, cervical length is measured transabdominally (TAUS) during the routine second-trimester sonographic fetal anatomic survey: If the TAUS cervix is short or is not adequately seen, then a TVUS examination is performed for a definitive measurement; if the TAUS cervix is clearly imaged and long, then TVUS may be avoided [74]. Using this approach, approximately 60 percent of patients need both a TAUS and a TVUS to assure that >95 percent of patients with a short cervix on TVUS are detected [65]. As a result, this approach is neither time-saving nor cost-effective [75].

The poorer performance of TAUS has been attributed to multiple factors, including (1) the bladder often needs to be filled to obtain a good image, resulting in elongation of the cervix and masking of any funneling of the internal os; (2) fetal parts can obscure the cervix, especially after 20 weeks; (3) the distance from the probe to the cervix results in degraded image quality; and (4) obesity and manual pressure interfere with the image [68].

Transvaginal ultrasound technique — The basic steps for the TVUS technique are:

●The patient should empty her bladder prior to the examination.

●Ultrasound gel is placed on a transvaginal probe before covering it with a specialized probe cover or condom, and then more ultrasound gel is placed on top of the cover. If the membranes are ruptured, both the cover and the gel should be sterile.

●With the real-time image in view, the transducer is gently inserted into the anterior fornix until the cervix is visualized while avoiding excessive pressure on the anterior cervical lip. The image of the cervix is enlarged to fill at least one-half of the ultrasound screen and oriented so cephalad is to the left of the screen. Fetal membranes in the cervical canal or beyond the cervix should be noted, if present.

●The amniotic fluid in the lower uterine segment is assessed and then the lowest edge of the empty maternal bladder. The internal os is then located, often just below this edge.

●The appropriate sagittal long-axis view for measuring cervical length includes the usually V-shaped notch at the internal os, the triangular area of echodensity at the external os, and the endocervical canal, which appears as a faint line of echodensity or echolucency between the two (figure 1). Excess pressure on the cervix can artificially increase its apparent length. This can be avoided by first obtaining an apparently satisfactory image, withdrawing the probe until the image blurs, and then reapplying only enough pressure to restore the image (image 1).

●Cervical length is represented by the line made by the interface of the mucosal surfaces (the closed portion of the cervix). It is usually the distance between calipers placed at the notches made by the internal os and external os. If the internal os is open (image 2), cervical length is measured from the tip of the funnel to the external os (figure 1). Cervical length should only be determined from images in which the lowermost edge of the empty maternal bladder and the internal os and external os are visible and when the anterior and posterior lips of the cervix are of approximately equal thickness. If the cervix appears asymmetric (thin anteriorly and thicker posteriorly), this suggests excessive probe pressure.

●At times, the cervical canal is curved. In these cases, the length of the cervix can be measured in either of two ways:

•The length of a single, straight line from the internal to external os can be measured.

•The sum of two separate, straight lines joined at an angle along the curved length of cervix is determined: This sum is used for the cervical length if the distance between the angle and a straight line from the internal to external os is >5 mm (image 3) as it may provide a more accurate measurement [7].

We avoid tracing the cervical canal because it introduces unpredictable operator variation. A curved cervix usually means a long cervix and thus a low risk for spontaneous preterm birth, while a short cervix is usually straight.

●When three measurements have been obtained that satisfy measurement criteria and vary by less than 10 percent, the shortest of these is chosen and recorded as the "shortest best." Choosing the shortest of three excellent images reduces interobserver variation. We do not determine the best measurement by image quality because this introduces an unpredictable variable.

●Moderate to firm manual transabdominal pressure applied across the fundus in the direction of the uterine axis for 15 seconds [76] can aid the examination by revealing a "dynamic" cervix (ie, the development of short cervical length in a cervix seemingly initially of normal length) [7,77]. It is important to allow at least five minutes for the total examination and a couple of minutes between the gentle application of fundal pressure and recording the presence of a short cervix as it takes time for development of dynamic and/or "transfundal pressure elicited" changes in the cervix [78].

If a short (or shorter) cervical length is seen after application of fundal pressure, the length of the residual closed portion of the cervix is taken three times, with the shortest length recorded in millimeters as the best estimate of the true length of the cervix. This length best correlates with duration of pregnancy. Only one measurement should be reported: the shortest best cervical length (mm) of all measurements taken.

Pitfalls in measuring cervical length — The following pitfalls can lead to suboptimal measurement of cervical length, typically resulting in overestimation:

●Excessive pressure – Placing excessive pressure on the cervix during the examination is a common mistake in performing TVUS. This creates an artificially longer cervix due to compression of the anterior cervical lip and lower uterine segment. As discussed above, this may be avoided by withdrawing the probe when the internal os and external os are visualized until slight blurring occurs, and then the probe is inserted slightly until a clear image returns. The anterior and posterior lips of the cervix should be of approximately equal thickness (figure 1).

●Not allowing enough time to view dynamic changes – Measuring cervical length too quickly is common and can result in an inaccurate measurement. It is important to allow adequate time (approximately five minutes) for any effects of transient pressure on the cervix to resolve.

●Uterine contractions – Contractions during the examination can cause a false impression of a long cervix. If the internal os is not clearly visualized and a contraction is present, the sonographer needs to wait until the contraction resolves before the cervical length can be measured accurately. Contraction of the lower uterine segment can mimic funneling with a normal residual cervical length. Uterine contractions occur more often after bladder emptying [79].

●Underdevelopment of the lower uterine segment – As discussed above, before 14 weeks, it is more difficult to differentiate between the lower uterine segment and true cervix as the pregnancy has not yet expanded to the whole uterus. Placenta previa may create this same problem, resulting in an artificially increased cervical length.

If the lower uterine segment is underdeveloped, it can be difficult to identify the true internal os, and some myometrium may be included in the cervical length measurement. This should be suspected when the cervix appears longer than 50 mm or the internal os is cephalad above the bladder reflection [14]. A difference in echotexture between myometrium and true cervical stroma often can be appreciated during real-time scanning and provides a means for differentiating between the two structures.

●Prior cervical surgery – Prior cervical surgery may alter the appearance of the cervix, making the identification of measurement landmarks difficult.

●Air bubbles – Hasty placement of lubricant into the transducer cover may generate small air bubbles that create a poor image.

Other cervical findings — During the ultrasound examination, additional findings associated with spontaneous preterm birth may be noted.

●Change in length over time – In patients diagnosed with a short cervix, a stable or longer cervical length at a subsequent examination is associated with a lower risk for spontaneous preterm birth than initially predicted, while a shorter cervical length increases the risk of spontaneous preterm birth [80-82].

●Separation of the membranes from the decidua and debris/sludge (hyperechoic matter in the amniotic fluid (image 4)) close to the internal os suggest subclinical infection and an increased risk of spontaneous preterm birth [83-85]. The composition of the debris is unclear; it may be a blood clot, meconium, vernix, or cellular material related to infection/inflammation [86].

●Funneling is the protrusion of the amniotic membranes into the cervical canal. Funneling has been variably defined according to the depth of protrusion [1] and/or the ratio of the funnel depth to the length of funnel plus the remaining closed cervix [77]. As the cervix effaces, the relationship between the lower uterine segment and the axis of the cervical canal also changes and is described according to the shape of the letters "T," "Y," "V," and "U" (mnemonic: Trust Your Vaginal Ultrasound) (figure 2) [57]. "T" represents the normal relationship of the area where the endocervical canal meets the uterine cavity, whereas "U" represents almost complete effacement and signifies the highest risk for spontaneous preterm birth. Representative endovaginal ultrasound images that display these changes are shown in the following ultrasounds (image 5A-C).

The length of the funnel is often uncertain because landmarks, such as the shoulder of the internal os, may not be distinct; therefore, we do not measure funnel length or use it for clinical management. In fact, while funneling is associated with a short cervix, it is not an independent predictor of preterm labor risk when the closed length of the cervical canal is considered [7,77]. As discussed above, when funneling is present with a normal residual cervical length, it is usually related to a contraction of the lower uterine segment and has little to no clinical significance.

●Assessment of cervical tissue density, cervical axis relative to the uterine corpus, and other cervical characteristics does not significantly improve predictive value for spontaneous preterm birth over cervical length alone [7,77,87].

Online resources — The Cervical Length Education and Review (CLEAR) program is available online and provides educational lectures, optional examinations, and scored image reviews to teach clinicians a standard, accurate method for measuring cervical length. An online tutorial is also available from the Fetal Medicine Foundation (United Kingdom).

Quality assurance — TVUS should be performed in accordance with all of the technical steps described above to obtain adequate measurements of cervical length. With proper technique, intra- and interobserver variation are <10 percent. Certification may be obtained via the CLEAR program, which integrates an online course with an online examination and image review (clear.perinatalquality.org) or via the Fetal Medicine Foundation, which also provides an online course (fetalmedicine.org/education/cervical-assessment).

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Preterm labor and birth".)

SUMMARY AND RECOMMENDATIONS

●Significance of short cervical length – A decrease in cervical length to ≤25 mm before 24 weeks is predictive of spontaneous preterm birth, and this risk increases as cervical length decreases. By contrast, a gradual decline in cervical length after 32 weeks can be normal and not predictive of spontaneous preterm birth. (See 'Rationale for measuring cervical length' above.)

●Screening – We suggest routine TVUS screening for short cervix (Grade 2B) since appropriate interventions to reduce the risk of spontaneous preterm birth are available. The following algorithm summarizes our approach (algorithm 1). (See 'Clinical approach' above.)

●Procedure for measurement of cervical length – Cervical length is measured by determining the length of closed cervix between the internal os and external os on transvaginal ultrasound (TVUS). It should only be determined from images in which the lowermost edge of the empty maternal bladder and the internal os and external os are visible and when the anterior and posterior lips of the cervix are of equal thickness (figure 1). (See 'Procedure for sonographic measurement of cervical length' above.)

●Diagnosis of short cervical length – We make the diagnosis of a short cervix when TVUS cervical length is ≤25 mm before 24 weeks, regardless of the population (eg, prior preterm birth, no prior preterm birth, twin gestation). (See 'Diagnosis of short cervix' above and 'Procedure for sonographic measurement of cervical length' above.)

●Management of short cervical length

•Patients with a prior spontaneous singleton preterm birth are offered progesterone (vaginal [preferred], or intramuscular) when first seen for prenatal care, based on their past pregnancy history. Progesterone is started optimally at 16 weeks. If a short cervix is subsequently identified <24 weeks, they are offered the addition of a cerclage. (See 'Parous patients with a prior spontaneous preterm singleton birth' above.)

•Patients with a prior spontaneous twin preterm birth – The best approach to patients with a prior spontaneous twin preterm birth is controversial. Some studies have reported that a prior spontaneous twin birth <34 weeks is associated with an increased risk of spontaneous preterm birth in the subsequent singleton pregnancy. If the prior twin preterm birth was ≥34 weeks, we manage the subsequent singleton pregnancy the same as in patients with no prior preterm spontaneous birth. If <34 weeks, we offer progesterone supplementation starting at 16 weeks and monitor cervical length as described above for singletons. (See 'Parous patients with a prior spontaneous twin birth' above.)

•Patients with a singleton gestation and no prior spontaneous preterm births are offered vaginal progesterone to reduce the chances of preterm birth. Placement of a cerclage may be helpful in those with a very short cervix (TVUS cervical length <10 mm). (See "Progesterone supplementation to reduce the risk of spontaneous preterm labor and birth", section on 'Candidates for progesterone supplementation' and 'Nulliparous patients and parous patients with no prior spontaneous preterm singleton birth' above.)

•Patients with a multiple gestation (See "Twin pregnancy: Management of pregnancy complications", section on 'Approach to patients with a short cervix'.)