Cervical insufficiency has been traditionally defined as painless cervical dilation that leads to mid-trimester pregnancy loss in the absence of other causes (e.g., labor, trauma, major fetal anomaly, stillbirth). Using this definition, the incidence of cervical insufficiency in the general obstetric population is between 1/100 and 1/2000 births.1, 2, 3, 4 The most traditional definition requires that pregnancy losses be recurrent; unfortunately, the history-based definition of cervical insufficiency requires that a patient experience a pregnancy loss.
In recent years, the definition of cervical insufficiency has expanded to include women with a prior spontaneous preterm birth and evidence of cervical shortening (<25 mm) on transvaginal ultrasound. When this criterion is included in the definition, the incidence increases to 3–4% of the obstetric population.5
Clinical reports of mid-trimester pregnancy loss attributable to a primary cervical etiology appeared as early as the 17th century; the term “cervical incompetence” was first used in The Lancet in 1865.6 The contemporary concept of cervical insufficiency was not widely accepted until the middle of the 20th century after Palmer7 in 1948 and Lash8 in 1950 independently described interval repair of anatomic cervical defects associated with recurrent mid-trimester loss. Soon thereafter, Shirodkar9 in 1955, McDonald10 in 1957 and, later, Benson11 in 1965 described the mid-trimester cerclage procedures now utilized in obstetric practice. Other investigators reported success with alternate surgical procedures including the Wurm bridging procedure by Hefner,12 cone-electrocautery scarification of the external os by Barnes,13 and scarification with suture-closure of the external os by Baden.14
For the remainder of the 20th century, investigators focused primarily on pregnancy outcomes in cohorts managed with transvaginal (and less commonly, transabdominal) cerclage procedures. These cohorts were not referenced to appropriate control groups (i.e. women with a similar presentation randomized to no intervention), but rather only compared to the cohorts’ own obstetric histories or data from analogous reports. In short, the 20th century literature on women with a history of cervical insufficiency has been largely a chronicle of surgical methods to correct anatomic disruption of the internal os in women who had experienced recurrent painless dilation and mid-trimester birth.
The syndrome of spontaneous preterm birth
Spontaneous preterm birth may be best characterized as a syndrome comprising several anatomic and related functional components.15 These include the uterus and its myometrial contractile function (e.g., preterm labor), decidual activation and loss of chorioamnionic integrity (e.g., preterm membrane rupture) and, finally, diminished cervical competence, from either a primary anatomic defect or an early pathologic cervical ripening (e.g., cervical incompetence or cervical insufficiency). In a particular pregnancy, a single anatomic feature may appear to predominate, even though it is more likely that most cases of spontaneous preterm birth result from the interaction of multiple stimuli and pathways which culminate in the recognized clinical syndrome. Nevertheless, the relative importance of these (and possibly other as yet unidentified) components can vary, not only among different women, but also in successive pregnancies of an individual patient.
The normal cervix
The cervix plays an essential role in maintenance of a normal pregnancy; disruption of the normal anatomic structure or biologic function of the cervix may foretell preterm delivery. The cervix is composed primarily of fibrous connective tissue; this fibrous band is felt to be the chief mechanical barrier against the loss of the enlarging products of conception. The cervix and its mucous glands also play an important immunologic role in preventing organisms from ascending into the normally sterile intrauterine environment.
Early in pregnancy the cervix begins to undergo structural changes with increased vascularity, stromal hypertrophy, glandular hypertrophy and hyperplasia, and structural changes of the extracellular matrix.16, 17, 18, 19 Conformational changes in cervical collagen, with alterations in the number and types of cross-links between collagen triple-helices, lead to a steady increase in tissue compliance throughout pregnancy. Those who have alterations in the process, due heritable defects in collagen and elastin synthesis or assembly, have an increased risk of cervical insufficiency.20, 21, 22, 23, 24
The biologic continuum of cervical insufficiency
As early as 1962 Danforth and Buckingham suggested that cervical insufficiency was not an all or none phenomenon.25 Rather, it comprised degrees of insufficiency, and combinations of factors could cause “cervical failure”. This hypothesis never gained wide acceptance in spite of the obvious heterogeneity observed in clinical practice. Cervical insufficiency was generally viewed as dichotomous (possibly because available treatment strategies were similarly devised).
Based on analyses of cervical tissue for collagen/muscle ratios and recognizing the contribution of past traumatic disruption of cervical continuity, these investigators proposed a classification scheme to explain the clinical syndrome of cervical insufficiency.25 In the proposed classification, one group of patients had ostensibly normal cervical tissue, whose integrity as an anatomical fibrous ring had been previously damaged. Such defects were usually the result of antecedent obstetric trauma or complications of dilation and curettage. These might even be concealed by a normal-appearing external os and ectocervix. The second group possessed an abnormally low collagen-muscle ratio that would compromise its mechanical function and lead to premature dilation. This so-called “muscular cervix” was probably a congenital problem and would manifest as a poor reproductive history in women with no prior cervical injury. The third group comprised women who had no history of antecedent trauma and who also had normal collagen-muscle ratios, but whose obstetric histories mimicked those of groups 1 and 2, presumably from stimuli which cause premature cervical ripening.
In a subsequent report these investigators analyzed cervical biopsies taken from postpartum women and compared them to hysterectomy specimens from non-pregnant patients.26 Pregnancy was associated with increased water content, a marked decline in collagen and glycoprotein, and increased glycosaminoglycans. The cellular and biochemical changes suggested that cervical dilation in pregnancy is a dynamic process, and this might explain why a woman could have a pregnancy outcome consistent with cervical insufficiency in one pregnancy, but then without treatment, have a subsequent term birth. Presumably, the factor(s) inciting the pathologic cervical changes would vary among pregnancies. Women with a more muscular cervix might have an unusual susceptibility or lower threshold for the effects of the factors which precipitate the clinical syndrome of preterm birth.
These earlier observations were enlarged by Leppert and colleagues27 who reported an absence of elastic fibers in the cervixes of women with clinically well-characterized cervical insufficiency on the basis of their reproductive history. Conversely, cervical biopsy specimens from women with normal pregnancies showed normal amounts and orientation of these elastic fibers. Supporting these microscopic observations was the biochemical finding of lower desmosine levels in the women with cervical insufficiency, indicating lower elastin content in the specimens. Rechberger and colleagues28 also compared cervical biopsy specimens among non-pregnant controls, women in the mid-trimester with clinically defined cervical insufficiency and normal postpartum gravidas. Compared to normal postpartum patients, they found increased collagen extractability and collagenolytic activity in women with cervical insufficiency, suggesting a high collagen turnover characterized by higher proportions of newly synthesized collagen with lower mechanical strength. It is unknown whether these microstructural and biochemical phenomena were congenital, acquired from previous trauma or the result of other pregnancy-associated processes. Collectively, these biochemical and ultrastructural findings support the variable, and often unpredictable, clinical course of women with a history of cervical insufficiency.
Although the traditional paradigm has depicted the cervix as either competent or insufficient, recent evidence, including clinical data29, 30, 31, 32 and interpretative reviews33, 34, 35 suggest that, as with most other biologic processes, cervical competence is rarely an all or none phenomenon and most likely functions along a continuum of reproductive performance. Although some women have tangible anatomic evidence of poor cervical integrity, most women with a clinical diagnosis of cervical incompetence have ostensibly normal cervical anatomy. In a proposed model of cervical competence as a continuum, a poor obstetric history results from a process of premature cervical ripening, induced by a myriad of underlying factors, including subclinical infection, inflammation, local or systemic hormonal effects, or even genetic predisposition. If and when cervical integrity is compromised, other processes may be stimulated, appearing clinically as other components of the spontaneous preterm birth syndrome (i.e., premature membrane rupture, or preterm labor). A decision as to whether diminished cervical competence arises via primary endogenous mechanical deficiencies or exogenous factors, might define the optimal therapy. A more cogent approach to the concept of cervical competence is to view the cervix as an interdependent participant in the multifactorial model of the spontaneous preterm birth syndrome. The pragmatic clinical question that emerges from this paradigm is whether an individual patient has a significant (and presumably treatable) component of diminished cervical competence.
While the historic concept of the diagnosis and treatment of cervical incompetence often includes women with past cervical trauma from birth-associated lacerations, operative injury or cervical amputation, the prevalence of these antecedent events appears to be decreasing in contemporary practice. Obstetric care and surgical technique have improved, clinicians recognize the importance of gradual cervical dilation prior to curettage and cervical amputation is rarely performed. Cervical damage from trauma may be visible or palpable on a careful examination of the cervix before or during the early part of pregnancy, and if detected, should be managed with surgical repair if associated with a prior poor obstetric history or if the defect is believed to involve the internal cervical os.
Diethylstilbestrol (DES) exposure in utero
DES, a synthetic estrogen designed to supplement a woman’s natural estrogen and prevent miscarriage or preterm delivery, was used by an estimated 5−10 million people between 1938 and 1971.36 As this medication has not been used since the early 1970s, women of reproductive age now have not been exposed in utero. However, approximately two-thirds of women exposed to DES in utero have reproductive tract anomalies and a significantly increased risk of cervical insufficiency.37, 38, 39
Mechanical cervical dilation
Mechanical cervical dilation is commonly performed during mid-trimester dilation and evacuation procedures. Assessing the effect of mechanical cervical dilation on the risk of cervical insufficiency is difficult. First, an appropriate comparison group is difficult to determine. The most common comparison groups used are women with no prior pregnancies and women with a prior term birth. Perhaps the most appropriate comparison group would be women with a mid-trimester induction of labor with no associated surgical procedure; this cohort would most closely resemble women undergoing mid-trimester dilation and evacuation in both sociodemographics and pregnancy history. We were unable to identify a study utilizing this comparison group. Second, ascertainment of the outcome of cervical insufficiency and the exposure is difficult. This type of study requires a large cohort, typically large population-based databases. Cervical insufficiency may not be recorded in these databases as a diagnosis, and preterm delivery is frequently used as a surrogate marker. Additionally, many have used birth certificate databases, which may include only live births after 20–24 weeks, excluding those with a mid-trimester loss who did not reach this gestational age and may have a true diagnosis of cervical insufficiency. Large databases may only record gestational age and delivery, rather than type of procedure, making ascertainment of the exposure (mid-trimester mechanical cervical dilation) difficult.
In a well-performed review by Atrash et al.40 that considered the above issues, dilation and evacuation procedures were associated with an increased risk of mid-trimester spontaneous loss. A more recent systematic review and meta-analysis suggested a slight increase (odds ratio 1.36, 95% CI 1.24–1.50) in the risk of preterm delivery after induced termination of pregnancy which was increased after multiple induced terminations (OR 1.93, 95% CI 1.28–2.71).41 In 2013, a study using the Scottish Morbidity Record and Scottish Stillbirth and Death Survey demonstrated that: (1) the risk of early spontaneous preterm delivery increased with the number of prior terminations of pregnancy and (2) this risk decreased over time in conjunction with any increases in medical abortions and cervical pretreatment (ripening) prior to surgical abortions.42 By the year 2000, the association of termination of pregnancy and early spontaneous preterm delivery in the following pregnancy was not apparent. Thus, while forced mechanical dilation of the cervix may be associated with a slight increase in the risk of preterm delivery and/or cervical insufficiency, newer operative techniques may mitigate that risk.
Loop electrosurgical excision procedures, cervical conization, and cervical dysplasia
Prior treatment for cervical dysplasia using loop electrosurgical excision procedures (LEEP), cold-knife conization, or laser conization to remove a portion of the cervix is plausibly a risk factor for cervical insufficiency. For similar reasons as examining the effect of mechanical dilation, estimating the effect of LEEP/conization is difficult: a randomized control trial is not possible, identifying a proper control group is difficult, details of the procedure are unlikely to be available in a large database, and preterm birth is frequently used as a surrogate marker for cervical insufficiency.
A multitude of retrospective studies have been published with varying results.43, 44, 45, 46, 47, 48, 49, 50, 51, 52 A systematic review and meta-analysis of 27 studies was performed in 2013.53 All studies that compared women who had undergo an ablative or excisional conization for cervical intraepithelial neoplasia or stage IA1 cervical cancer to an untreated group and examined the outcome of preterm birth were included. In eight studies with a total of 3141 subjects, LEEP was associated with an increase preterm delivery <37 weeks in a relative risk of 1.70 (95% CI 1.24–2.35). In eight studies with 28,378 subjects, cold knife conization was associated with an increased risk of preterm delivery with a pooled relative risk of 2.59 (95% CI 1.80–3.72). Laser conization and ablation were not associated with an increased risk of preterm delivery (pooled RR 1.71, 95% CI 0.93–3.14 and RR 0.87, 95% CI 0.63–1.20). In another systematic review and meta-analysis, Kyrgiou examined the effect of excisional procedures on second trimester miscarriage rates; in this analysis of eight studies that included a total of 16,558 excisional procedures, second trimester miscarriage was increased after an excisional procedure (pooled RR 2.60, 95% CI 1.45–4.67).54
As cervical excisional procedures purportedly increase the risk of cervical insufficiency through mechanical/structural alteration of the cervix, one would presume that either the quantity of tissue removed from the cervix would be associated directly with the risk of preterm delivery and/or that cervical length after excisional procedure would be predictive of preterm delivery risk. Indeed, the few published studies that have examined the amount of tissue excised at the time of the procedure and the risk of preterm delivery did find a correlation between the two.52, 55, 56
Studies examining the mid-trimester cervical length consistently show that, compared to women with no prior surgical procedure, women with a prior excisional procedure had a shorter cervical length and in some studies a higher incidence of short cervix (<25 mm or <30 mm, depending on the study).57, 58, 59, 60, 61 Miller et al. performed a multivariable analysis to assess the independent contributions of a cervical excisional procedure and a shortened cervix; they concluded that both were independently associated with the risk of preterm birth and that the increase in risk of preterm birth after an excisional procedure is not solely through a shortened cervix mechanism.60
Conner et al. proposed that the risk of preterm birth after excisional procedure may be associated with the biological mechanism of dysplasia.62 In a systematic review and meta-analysis, they considered the presence or absence of cervical dysplasia in the control group. When women with a LEEP were compared to women with either no history or an undocumented history of cervical dysplasia, LEEP was associated with an increased risk of preterm birth (pooled RR 1.86, 95% CI 1.58–2.21). When women with a history of a LEEP were compared to women with a history of cervical dysplasia, LEEP was not associated with an increased risk of preterm birth (pooled RR 1.08, 95% CI 0.88–1.33).
In sum, a history of a cervical excisional procedure is associated with a small increase in the risk of preterm birth, although the mechanism of the increased risk is unclear.
Prophylactic treatment based on risk factors
Cervical length screening and/or treatment for cervical insufficiency with cerclage on the basis of one or more of the above risk factors has not been proven effective and is not indicated.
Traditionally, cervical insufficiency is a clinical diagnosis based on the history of a spontaneous midtrimester loss associated with painless cervical dilation. The burden of the diagnosis of cervical insufficiency is on the physician managing the patient at the time of the mid-trimester loss. Other causes of midtrimester loss should be excluded (labor, stillbirth, abruption, trauma, fetal anomaly, genetic causes, and uterine anomalies). As multifetal gestations are at high risk of mid-trimester loss, a diagnosis of cervical insufficiency should not be made after a midtrimester loss of a multifetal gestation. Careful documentation of patient presentation (presentation with hourglassing membranes, vaginal discharge, vaginal spotting, rupture of membranes, presence or absence of contractions, presence or absence of fever) is necessary. A diagnosis of cervical insufficiency should not be made simply in the presence of additional risk factors (including a current multifetal gestation or prior midtrimester loss of multifetal gestation). Additionally, as records from prior deliveries are not always available, informing the patient of her diagnosis and counseling her prior to discharge from the hospital that treatments may be available early in future pregnancies to prevent recurrent pregnancy loss is of the utmost importance.
The difficulty of diagnosing cervical insufficiency by patient history alone has led physicians to seek other, more objective diagnostic measures. Various sonographic findings including cervical length, funneling at the internal os, and dynamic response to provocative maneuvers (e.g., fundal pressure) have been utilized to select women for treatment, generally cerclage. In most of these earlier reports, the sonographic evaluations were not blinded, leading to uncontrolled interventions and difficulty determining their value. In many instances the sonographic criteria for cervical incompetence were only qualitatively described and thus, may not be reproducible. Table 1 depicts published reports linking the findings from cervical sonography to a diagnosis of incompetence. Note that the diagnostic criteria are disparate and in many cases, not described in a quantitative fashion.
Table 1. Published reports of the sonographic diagnosis of cervical incompetence
Author, year (Ref)
Criteria for cervical insufficiency
Authors suggested each center establish their own criteria
Width of internal os <1.9 cm
13 received cerclage; included women with labor symptoms
Detached membranes at internal os building into a dilated cervical canal
40 received cerclage
Not explicitly stated; cervical canal width >8 mm implied
32 received cerclage for sonographic CI
Membrane prolapse >6 mm and short cervix
Previous MT loss
70 received cerclage
CL >40 mm (−2 standard deviations from the mean)
45% received cerclage
Not explicitly stated
5 underwent cerclage
Combination of membrane protrusion, progressive herniation, cervical
Retrospective cohort study; 51 received cerclage
Combination of cervical shortening, canal dilation & membrane
History of CI
2 case reports; both underwent cerclage
Progressive shortening and breaking of the chorioamnion
At risk or history of classic CI
Used fundal pressure as provocative maneuver
Dilation of the internal os with the membranes protruding into canal or funneling & cervical shortening
Used sonography to avoid cerclage in women with classic history; 12 of 19 underwent cerclage
>1 cm decrease in CL and/or funneling
Examined natural history of fundal pressure response
CL <10 mm or cervix dilated on physical examination
Transfundal pressure and other provocative maneuvers
Progressive cervical changes to a CL <26 mm
16 had prophylactic cerclage; postural test evaluated as a provocative maneuver
33% decrease in CL
Retrospective cohort study; elective vs. ultrasound-indicated cerclage
CL <20 mm with or without fundal pressure
All urgent cerclages
Urgent cerclage placed for sonographic indications
Breaking of amniotic fluid at internal os
CL <10 mm with or without fundal pressure
GA, gestational age; DES, diethystilbestrol; N/S, not stated; MT, midtrimester; CL, cervical length; CI, cervical incompetence
Subsequently large, blinded, and reproducible observational studies investigating the relationship between mid-trimester cervical length and the risk of preterm birth were published.29, 79, 80, 81, 82, 83 Iams and his colleagues in the NICHD Maternal-fetal Medicine Units Network completed a study of 2915 unselected women with a singleton pregnancy that underwent a blinded cervical sonographic evaluation at 22–24 weeks’ gestation.29 The relative risk of spontaneous preterm birth increased as cervical length decreased. In spite of this significant relationship, as a test for predicting spontaneous preterm birth <35 weeks at a cervical length cutoff of <26 mm (the population 10th percentile), the sensitivity was low (37%), and the positive predictive value was poor (18%). These findings were confirmed by Taipale and colleagues in an unselected, but generally low-risk Finnish population.80 Both groups concluded that cervical sonography was not a viable screening test for spontaneous preterm birth in a low-risk population.
Owen and colleagues examined the utility of cervical ultrasound as a predictor of spontaneous preterm birth at <35 weeks in high-risk women.83 Participants included gravidas with a prior spontaneous preterm birth at <32 weeks. Women believed to have cervical insufficiency (based on a clinical history) were not eligible. Beginning at 16–18 weeks of gestation, 163 gravidas underwent serial, biweekly sonographic evaluations until the 23rd week of gestation. Fundal pressure was also utilized as a provocative measure to determine whether dynamic cervical shortening could be induced. This study design permitted analysis of the shortest observed cervical length over time which also included both spontaneously occurring and fundal pressure-induced cervical length shortening. As in the previous studies by Iams29 and Taipale80the risk of spontaneous birth increased as cervical length shortened. However, in this high-risk population, the sensitivity increased to 69% and the positive predictive value to 55% at a cervical length cutoff of <25 mm (the 10th centile at the 16–18 weeks, intake evaluation). In other words, 55% of women with a prior preterm birth <32 weeks and a cervical length <25 mm at their first evaluation delivered prior to 35 weeks.
In sum, cervical sonography performs poorly as a screening test in low-risk women but it appears to have significant clinical utility in high-risk women, defined as a prior early spontaneous preterm birth. Whether cervical ultrasound has similar predictive values in other populations of at-risk women (e.g., DES, prior cervical surgery, uterine anomalies, multiple induced abortions, etc.) remains speculative, as it has not been well-studied. Some investigators have included women with these risk factors in study populations comprised primarily of women with prior spontaneous preterm birth, but the results could not been subcategorized because of small sample sizes.81
Use of cervical ultrasound in twin gestations has also been reported; however, the test characteristics, especially sensitivity and positive predictive value (<40%), appear to be generally lower than for women with a prior early spontaneous preterm birth.84, 85 Other case series have documented low (<10%) incidences of significant mid-trimester cervical length shortening in populations of multiple gestations.84, 86, 87, 88
Therefore, a diagnosis of cervical insufficiency may be made in a woman with a singleton pregnancy with a history of prior preterm birth (of a singleton gestation) and a mid-trimester (<24 weeks) cervical length <25 mm.
Occasionally, a woman will present with symptoms and physical findings that support an antepartum diagnosis of acute cervical insufficiency. This typically consists of vague symptoms of pelvic pressure, discharge, spotting, and/or increased urinary frequency. Physical exam demonstrates a cervical dilation of 1–4 cm and fetal membranes visible at or beyond the external os. These patients warrant monitoring for intrauterine infection (fever, uterine tenderness, leukocytosis) and preterm contractions/labor.
Diagnostic testsBecause the diagnosis of cervical insufficiency has been determined primarily from past reproductive performance and physical examination findings, the obvious limitations associated with the clinical diagnosis have prompted the search for sensitive and specific tests that could be applied in a prospective manner to women deemed at risk for cervical insufficiency, thus obviating recurrent pregnancy loss as a diagnostic criterion. Such a test would ideally provide a timely diagnosis and the potential for optimal therapeutic intervention.
To be clinically useful, a scoring system, or other test for cervical insufficiency should be able to accurately discriminate between women who have a condition amenable to a proven effective form of therapy and women with other causes of poor obstetric outcome. Even in the absence of effective therapies, such an assessment would be useful to target at‑risk populations, whose optimal treatments could be determined from controlled intervention trials.
Most of the earlier reported tests for cervical insufficiency were based on the functional anatomy of the interval os in the non-pregnant state and are of historical interest. In 1953, Rubovits and colleagues utilized an intrauterine balloon filled contrast media and imaged with a radiograph to discriminate the normal from the incompetent lower uterine segment and internal os.89 Mann, in 1959 reported a new method for the diagnosis of cervical incompetence which used an intrauterine balloon with a contrast agent and subjected to filling pressures of 200–260 mmHg.90 This, he believed, overcame the limitations of earlier diagnostic studies that utilized plain hysterosalpingography by measuring the internal os dimensions under pressure. As this evaluation yielded an internal os measurement, a continuous variable, he chose what appeared to be an arbitrary cutoff point to define insufficiency.
In 1973 Peterson and Keifer proposed another improvement after concluding that Mann’s technique described above would artificially cause cervical and uterine dilation using pressures was “fraught with danger.”91 Their apparatus was similar to Mann’s, but the intrauterine balloon had a second port open to the atmosphere and limited the intrauterine pressure. Studying 55 women with suggestive histories, these investigators excluded the diagnosis of insufficiency in 16 women when the maximum diameter of the cervical canal was less than 10 mm. They followed these women in successive pregnancies and observed 22 neonatal survivors, whereas their prior neonatal loss rate had been 52%; by inference, the etiology of these prior losses was not cervical insufficiency. In the women with cervical canal diameters ≥10 mm, prophylactic cerclage, either before or after conception, was performed, and these 26 women had 31 successful pregnancies. Since the subsequent loss rates could not be determined (the denominators were not reported), it was difficult to understand the magnitude of the benefit attributable to the test results or the cerclage. Other attempts at objective assessments included passage of a #8 Hegar dilator into the non-pregnant cervical canal without resistance92 and traction forces required to dislodge a Foley catheter whose balloon was placed above the internal os and filled with 2–3 mm of water.93 Subjectively effortless passage of the dilator or removal of the Foley balloon with <600 g of force would confirm an objective diagnosis.
More recently, in 1988, Kiwi and colleagues estimated the elastic properties of the non-pregnant cervix in two cohorts of women: 247 women with a poor obstetric history (defined as ≥3 first trimester abortions, ≥1 second trimester loss, ≥1 prior preterm birth, or some combination thereof) and 42 controls who may have had no more than two first trimester abortions, and no prior preterm births.94 Cervical elastance was derived from the pressure–volume slope of a 4 mm balloon placed in the endocervical canal and inflated to pressures as high as 150 mmHg. Although women in the poor obstetric history group had significantly lower elastance values than the controls, there was significant overlap between the two groups. They reported no subsequent pregnancy outcomes, proposed no clinically useful cutoff for their evaluation and could only suggest that such objective evaluation might ultimately prove to be clinically useful to accurately select patients for prophylactic cerclage.
In 1993 Zlatnik and Burmeister reported their experience with a cervical compliance score (analogous to an Apgar score) derived from the results of three other tests: hysterosalpingography, passage of a #8 Hegar dilator, and intrauterine balloon traction, performed in 138 non-pregnant women.95 Their histories included prior delivery <34 weeks following clinically diagnosed preterm labor or preterm membrane rupture, with or without antecedent bleeding. A small portion of their cohort had a questionable history of cervical insufficiency. Scores could range from 0 to 5, and women with low scores ≤2 were more likely to have been delivered at 27–34 weeks as compared to women with higher scores who were more likely to have been delivered in the mid-trimester at 14–26 weeks (p <0.01). In subsequent pregnancies, whose management and outcomes were ascertained by a mail-out questionnaire (83/138 responded), cerclage was recommended in most women with a score of 3 and in all women with a high score >3; 21/27 underwent surgery. Surprisingly, more women with scores ≥3, 78% of whom received cerclage, were delivered at 14–29 weeks' gestation (p = 0.07) casting doubt on its clinical utility to appropriately select patients for cerclage (under the presumption that cerclage is an effective therapy).
Others evaluated the utility of scoring systems to aid in patient selection for cerclage procedures, intending to increase the specificity of the clinical diagnosis. In 1976 Block and Rahal reported 31 pregnancies in 25 women who received prophylactic cerclage for a diagnosis of cervical incompetence.96 The score increased based on a greater number of clinical or historical indications for cerclage, and by inference, a more specific diagnosis of cervical insufficiency. Women with high scores had a longer cerclage-to-delivery interval as compared to women with lower scores. Women with higher scores also had higher overall pregnancy success rates with cerclage than did the cohort of women who had fewer indications and lower scores. In 1991 Ger and colleagues performed an analogous study of 47 women who received a McDonald cerclage based on their obstetric histories and physical examinations, but not necessarily for a clinical history of cervical insufficiency.97 Based on their obstetric histories and examination findings, scores could range from 1 to 5. Similar to Block and Rahal, they observed longer cerclage-to-delivery intervals in women with high scores >3 as compared to women with lower scores. While these observations support the multifactorial model of spontaneous preterm birth described earlier, these investigators could provide no practical recommendations.
All such attempts at providing an objective diagnosis of cervical insufficiency failed for a number of reasons, and none of these tests are in common use today. These reports generally suffered from a failure to evaluate standard test characteristics (i.e., sensitivity, specificity) against some reference standard for the diagnosis or another clinically relevant endpoint. They often utilized subjective criteria, caused patient discomfort, required ionizing radiation, were cumbersome and were generally only applicable in the non-pregnant state; many women at risk for cervical insufficiency present for care after a subsequent conception. Moreover, none of these tests could reasonably predict pregnancy-associated conditions that would lead to premature ripening and cervical dilation. Finally, since there is no universally applicable standard for the diagnosis of incompetence, and since the results of such tests were never evaluated and linked to a proved effective treatment, their clinical utility was, at best, theoretical.
The goal of screening is to identify those at risk of a disease before the onset of symptoms; the goal of screening for cervical insufficiency therefore is to identify women at risk of mid-trimester pregnancy loss or preterm delivery prior to the onset of contractions, rupture of membranes, or delivery. An effective screen program should (1) screen for a prevalent condition, (2) accurately identify those at risk for the condition, and (3) offer an effective treatment for the condition identified. The accepted method of screening for cervical insufficiency is transvaginal ultrasound;98, 99 who should be screened for cervical insufficiency using sonography remains controversial.
Transvaginal ultrasound is a screening test for cervical insufficiency that is safe, acceptable, and reliable. Those responsible for measuring transvaginal cervical lengths should undergo the Cervical Length Education and Review (CLEAR)100 program sponsored by the Society of Maternal Fetal Medicine through its Perinatal Quality Foundation and follow the standards for measurement in this program. When measured appropriately, transvaginal cervical length has an inter- and intra-observer variability of less than 10%.101
Transabdominal ultrasound is not considered an acceptable screening method for cervical insufficiency. Transabdominal ultrasound is less sensitive than transvaginal ultrasound, and image quality may be poorer than transvaginal ultrasound.102
Who should be screened?
In an unselected population of singleton pregnancies, a short cervical length (≤25 mm) has a positive predictive value for preterm birth prior to 35 weeks of only 17.8%; in other words, only 17.8% of women with a cervical length ≤25 mm at 22–24 weeks' gestation will deliver prior to 35 weeks.29 When women with singleton pregnancies and a prior preterm birth <32 weeks are considered, the positive predictive value of a cervical length <25 mm for preterm birth prior to 35 weeks increases to 75%, although the sensitivity remains low at 19%.83 In other words, in women with a singleton gestation and a prior spontaneous preterm birth <32 weeks, a cervical length <25 mm will only detect 19% of women who will go on to delivery at <35 weeks but of those who have a cervical length <25 mm, 75% of them will go on to deliver preterm (Table 2).
Table 2 Test characteristics of cervical length to predict preterm birth <35 weeks
Unselected singletons (nulliparous, multiparous, with or without history of preterm delivery)29
Singletons, prior preterm birth <32 weeks83
*This study reported the risk of preterm birth <34 weeks
Of note is the number of women needed to screen to identify one woman with a short cervix. In cohorts reporting screening of a general, low-risk population (singletons, no prior preterm birth) the incidence of a cervical length ≤25 mm was 0.89–2.0% and of cervical length ≤20 mm was 0.47–1.2%.105, 106 The incidence is higher in those with a prior preterm birth, approximately 12.5%.83
We are not aware of a randomized trial of screening for cervical insufficiency (i.e. women randomized to routine transvaginal cervical length measurement versus no screening that offers interventions and then measures rates of preterm birth between those randomized to routine screening versus no screening). In a retrospective cohort study examining the incidence of preterm birth before and after instituting a universal cervical length screening program, a universal cervical length screening program was associated with a decreased frequency of preterm birth prior to 37 weeks.106 In another retrospective study at a clinical center where universal screening was offered, women who declined screening were compared to those who accepted screening. Those who declined screening had a higher incidence of early preterm birth.105 Cost-effectiveness analysis suggests that universal screening of singleton fetuses is cost-effective when the treatment used for short cervix is vaginal progesterone.107
The low incidence of a short cervix and the controversy over appropriate treatment methods have led to debate over who should be screened (Table 3). The American College of Obstetricians and Gynecologists does not mandate universal screening; however, should screening be implemented, they recommend a protocol validated in a clinical trial to prevent preterm birth.98
Table 3 Summary of screening for cervical insufficiency by population
Yes (progesterone, pessary)
Singleton, prior preterm birth
No proven effective treatment at this time
The mainstay of cervical insufficiency treatment is the cerclage, which may be placed via vaginal or abdominal (cervico-isthmic) routes. History- and ultrasound-indicated cerclage are not an appropriate treatment for suspected cervical insufficiency in nulliparous women or in those with multifetal gestation. Alternative treatments for cervical insufficiency include vaginal progesterone and pessary.
Indications for cerclage
A history-indicated cerclage may be performed in singleton pregnancies in women with either (1) a history of mid-trimester loss characterized by painless cervical dilation without labor or abruption or (2) a history of a cerclage placement in a prior pregnancy for painless cervical dilation. Initially offered to women only with recurrent mid-trimester losses, a cerclage is now frequently offered after a single mid-trimester loss.108 A history-indicated cerclage is typically performed at 12–15 weeks of gestation after most spontaneous abortions have occurred so as to avoid complications of early spontaneous loss. It also permits a thorough sonographic evaluation of the fetus to rule out many severe or life-threatening anomalies. In selected cases, prenatal diagnosis using chorionic villus sampling can also be performed prior to the surgery.
An ultrasound-indicated cerclage may be placed in a woman with a prior preterm birth with a cervical length ≤25 mm. This type of cerclage is typically placed at 16–22 weeks, the typical time period for cervical length screening. An ultrasound-indicated cerclage is not indicated in women with a short cervix but no prior preterm delivery.98 An ultrasound-indicated cerclage may be more difficult to perform than a history-indicated cerclage due to cervical shortening.
Physical exam indicated
A physical exam-indicated cerclage may be placed when a woman presents with painless cervical dilation up to 23−24 weeks' gestation. Cerclage may be placed up to 4-cm dilation and may be considered with more advanced dilation; techniques to enhance visualization of the cervix in the presence of bulging membranes include amnioreduction, Trendelenburg position, use of a Foley catheter to replace the membranes into the lower uterine segment, and instillation of normal saline (up to 1 L) into the urinary bladder to displace the lower uterine segment. Prior to placement of an exam-indicated cerclage, the patient should be evaluated for evidence of active labor (manifested by contractions and progressive cervical change), intrauterine infection (fever, fundal tenderness), and abruption (vaginal bleeding).
Cervicoisthmic (abdominal) cerclage
Rarely a patient presents who is a viable candidate for a cervicoisthmic cerclage. This procedure is reserved for women who have failed a transvaginal cerclage; however, it is conceivable that some women failed transvaginal cerclage (with or without subsequent cervical damage) because they did not have cervical incompetence as the cause for their prior reproductive adversity. Because of the associated morbidity, a patient should be selected for cervicoisthmic cerclage only after a careful review of her history and a thorough clinical assessment indicates that the prior (failed) transvaginal cerclage was clinically indicated and that placement of another transvaginal cerclage is technically not feasible because of the magnitude of the anatomic deficiency. Primary cervicoisthmic cerclage might be considered in a patient with a typical history of incompetence and no prior transvaginal cerclage if the cervix is surgically or congenitally absent.
Contraindications to cerclage
A cerclage is not recommended in pregnancies complicated by known lethal fetal anomalies or stillbirth as no fetal benefit is expected to be gained by extending the length gestation in these pregnancies. Cerclage placement in pregnancies complicated by twins may increase the risk of preterm birth, even in the presence of a short cervix. Therefore, cerclage is not recommended in twin pregnancies.108, 109, 110 Cerclage may be harmful in the setting of intramniotic infection, active labor, preterm ruptured membranes, or active bleeding and therefore is not recommended.
Efficacy of cerclage placement
The efficacy of history-indicated cerclage is difficult to evaluate due to the above described difficulties in diagnosing cervical insufficiency. Three randomized trials of singleton gestations have evaluated the efficacy of cerclage in women with a possible history cervical insufficiency.111, 112, 113 The largest trial (n = 1292) was conducted by the Royal College of Obstetricians and Gynaecologists from 1981 to 1988 in 12 countries.113 The patient population was heterogeneous with at least six risk-factor subgroups. The risk of preterm birth prior to 33 weeks was significantly lower in those that were assigned to cerclage compared to expectant management (13% versus 17%, p = 0.03), although women in the cerclage group also received more tocolytics, were hospitalized more frequently, and suffered from puerperal fever more often. A subgroup analysis of this study suggests that cerclage was only beneficial when placed after three or more spontaneous preterm births. However, most obstetricians are loath to wait for three losses and intervene after only one, despite a lack of evidence suggesting benefit.114
A meta-analysis of randomized trials in women with a prior spontaneous preterm birth assigned to either history-indicated cerclage or cervical length screening with cerclage if the cervix shortens determined that history-indicated cerclage did not perform better than cervical length screening.115 Additionally, the majority of women who underwent cervical length screening did not develop a short cervix. Therefore, the authors concluded that women with a history of prior preterm birth can be monitored with cervical lengths rather than undergo history-indicated cerclage.
In summary, history-indicated cerclage may not benefit most women. The preferred candidate for a history-indicated cerclage is a patient who has had recurrent mid-trimester loss consistent with cervical insufficiency. Those who had an early preterm delivery after cerclage for cervical shortening or in those whom a cerclage could not be placed after cervical shortening may also be candidates for a history-indicated cerclage.
Ultrasound-indicated, prior preterm birth
In women with a prior preterm birth, cerclage for a cervical length <25 mm is efficacious. Owen and colleagues performed the largest US trial, in which 1014 women with a prior spontaneous preterm birth between 16 and 34 weeks underwent serial cervical screening.5 Of the women screened, 318 women developed a cervical length <25 mm and 302 were randomized: 149 to cerclage and 153 to no cerclage. Women randomized to cerclage had a decreased risk of preterm birth prior to 37 weeks (45% versus 60%, p = 0.01), previable birth (<24 weeks, 6.1% versus 14%, p = 0.03), and perinatal death (8.8% versus 16%, p = 0.046).
Berghella et al. performed a meta-analysis of cerclage for short cervical length. Only trials that randomized women with a singleton gestation, prior spontaneous preterm birth, and short cervix to either cerclage or no cerclage were evaluated. Although several trials included other women (i.e. no prior preterm birth or twins), only data for women meeting the above inclusion criteria were evaluated. Four of the studies only included women with a preterm birth prior to 32–34 weeks; only one study included women with a prior preterm birth up to 36 weeks. The meta-analysis demonstrated a lower risk of preterm birth at any gestational age (<37, <32, <28, <24 weeks) in women receiving a cerclage with a cervical length <25 mm. Cerclage in this population was also associated with decreased perinatal mortality (pooled risk ratio 0.64, 95% CI 0.45–0.91). The authors calculated that 20 cerclage procedures are needed in this patient population to prevent one perinatal death.
Intramuscular 17-alpha-hydroxyprogesterone (17-OHP) also decreases the risk of preterm birth in women with a prior preterm birth.116 There does not appear to be an additive benefit to 17-OHP and cerclage,117, 118 nor is it known which intervention is superior for women with prior preterm birth. As neither intervention is 100% effective and physicians are also currently unable to predict who will respond to cerclage or 17-OHP, many women with a history of prior preterm birth will receive both interventions. Some physicians reserve cerclage for those with a prior preterm birth <34 weeks, as the majority of randomized trials were limited to this patient population.
Ultrasound-indicated, no prior preterm birth
Placement of a cerclage is not recommended in women with a short cervix and no history of preterm birth.98 In an individual patient meta-analysis of women with asymptomatic cervical shortening, cerclage was not beneficial in reducing the risk of preterm birth prior to 35 weeks in the subgroup of women with no prior preterm birth (RR 0.84, 95% CI 0.60–1.17).109
Women who present with acute cervical insufficiency, typically defined as a mid-trimester cervical dilation of at least 2 cm and no other predisposing cause (labor, infection, bleeding, rupture of membranes) are often considered for a physical-exam indicated cerclage. Data on the efficacy of this procedure are limited, and typically are reported as case series or cohorts.
Aarts and colleagues reviewed eight series published between 1980 and 1992 comprising 249 patients who received a physical-exam indicated mid-trimester cerclage and estimated a mean neonatal survival rate of 64% (reported range 22–100%).119 Novy and colleagues published a series of 35 cases of acute cervical insufficiency (cervical dilation 2–5 cm); 19 women received cerclage, and 16 were managed with bed rest.120 Neonatal survival was 80% in the cerclage cohort versus 75% in the bed rest group. In another report using a similar study design, Olatunbosun and colleagues documented a longer mean gestational age at delivery compared to bed rest (p=0.001), although neonatal survival was not significantly different (p = 0.3).121 The Global Network for Perinatal and Reproductive Health reported a cohort of women who received either cerclage or expectant management for acute cervical insufficiency presenting between 140/7 and 256/7 weeks.122 Women who received a cerclage tended to present earlier than those who were expectantly managed (19.1 versus 23.1 weeks); despite this difference, women receiving a cerclage were more likely to deliver at a later gestational age (33 versus 25.9 weeks, p = 0.003) and have a longer time from presentation to delivery (12.4 versus 1.6 weeks, p <0.001).
A small randomized clinical trial evaluated emergency (i.e. physical exam-indicated) cerclage plus bed rest versus bed rest alone in 23 women who presented with cervical dilation and membranes prolapsing to or beyond the external os prior to 27 weeks of gestation.123 Both singleton and twin gestations were eligible; however, no information on the amount of cervical dilation was reported, and so it is not known whether the groups were comparable in this important aspect. They observed a longer mean interval from presentation to delivery in the cerclage group (54 days versus 20 days, p = 0.046). The neonatal survival rate was 9 of 16 with cerclage and 4 of 14 in the bed-rest group. Although the survival differences were not statistically significant, there were significantly lower rates of neonatal composite morbidity (which included death) in the cerclage group (10 of 16 versus 14 of 14, p = 0.02). Their sample size was not large enough to assess cerclage efficacy in singletons versus twins, but in this rare circumstance, it may be reasonable to consider a physical exam-indicated cerclage for women with twins who present with acute cervical insufficiency.
Mays and colleagues performed amniocentesis in 18 women who presented with this syndrome and analyzed the amniotic fluid for glucose, LDH, Gram stain, and culture.124 An additional seven women were managed with cerclage after refusing amniocentesis. Clinical decisions were made based on amniotic fluid biochemistries, and low glucose and elevated LDH, which were immediately available, were sufficient for a diagnosis of subclinical infection. Of 11 women who underwent cerclage with no evidence of subclinical infection, the neonatal survival was 100%, and the mean latent phase duration from presentation to delivery was 93 days. Of the seven women with abnormal biochemistries in whom cerclage was withheld, no neonatal survivors were observed, and the mean latent phase was 4 days. Recognizing that at least a portion of the seven women who declined amniocentesis, but who received emergent cerclage, also had subclinical infection, it was predictable that the mean latent phase in this cohort was intermediate (17 days) as compared to the groups with amniotic fluid analyses. These investigators suggested that amniocentesis could aid in selecting candidates for emergent therapeutic cerclage. Although these findings were provocative, the uncontrolled study design did not substantiate that cerclage is effective in these cases, but it demonstrated that acute cervical insufficiency is frequently associated with intrauterine infection.
Alternatives to cerclage
As not all patients are candidates for cerclage, alternative, non-surgical approaches have been sought.
All women with a prior spontaneous preterm birth between 16 and 366/7 weeks should have recommended weekly intramuscular 17-alpha-hydroxyprogesterone 250 mg from 16 to 36 weeks of gestation.98 This intervention has a demonstrated 30% reduction in the risk of recurrent preterm birth.116
Progesterone may also be given vaginally in women with a short cervix. One large, multicenter randomized, placebo-controlled trial of 458 singletons with a short cervix on ultrasound (10–20 mm) demonstrated a decreased risk of preterm before 33 weeks (RR 0.55, 95% CI 0.33–0.92).125 This study also demonstrated a decreased risk of neonatal morbidity and mortality (RR 0.57, 95% CI 0.33–0.99). A meta-analysis that included five randomized trials including women with a short cervix on ultrasound demonstrated that vaginal progesterone for these women decreased the risk of preterm birth prior to 34 weeks or perinatal death (RR 0.66, 95% CI 0.52-–0.83); the number needed to treat to prevent one case of preterm birth prior to 34 weeks or perinatal death was 11.126 In women with a short cervix (≤20-mm) who are not a candidate for cerclage (i.e. no prior spontaneous preterm birth) or decline cerclage, vaginal progesterone is a treatment option.
Investigators in Europe and the United States have studied vaginal pessaries for the treatment of the incompetent cervix.127 In 1961, Vitsky proposed the mechanism whereby a lever pessary (Smith, Hodge or Risser design) might be an effective treatment for the incompetent cervix.128 A vaginal pessary would displace the cervix posteriorly and shift the gravitational effects of the expanding uterine contents off the internal os and onto the anterior lower uterine segment.
Data around pessary use are conflicting. A large randomized trial conducted in Spain for singletons with a cervical length <25 mm in the midtrimester compared Arabin pessary to expectant management. This study suggested a dramatic reduction in the risk of preterm birth prior to 34 weeks (27% versus 6%, p <0.001) and prior to 28 weeks (8% versus 2%).129 However, another large multicenter, multinational trial found no benefit of pessary for preterm birth prevention <34 weeks (OR 1.12, 95% CI 0.75–1.69) or <28 weeks (OR1.71, 95% CI 0.89–3.29).130
Pessary has also been investigated for use in twin pregnancies. A subgroup analysis of a randomized trial of pessary for preterm birth prevention in twins suggested potential benefit for those with a cervix <38 mm.131 This treatment should be studied further prior to universal implementation of this intervention; studies are ongoing in the US and in Europe (www.clinicaltrials.gov).
Cerclage procedures still in common use today include those originally described by Shirodkar,9 McDonald,10 and Benson,11 although various modifications to each have gained wide acceptance.
Although McDonald10 and Shirodkar9 originally reported their surgical techniques in temporal proximity, most clinicians now favor the McDonald method for several reasons. Because it does not require mucosal dissection and elevation to expose the cervix at the level of the internal os, it is easier to perform and is associated with less blood loss. The circumferential suture is tied on the ectocervix and easily accessible, which facilitates later cerclage removal. Although both methods have their proponents, there are no controlled comparative data on treatment efficacy. The findings of uncontrolled cohort series suggest that success rates (usually defined as perinatal survival) are similar and neither technique has an obvious advantage.132, 133
To place a prophylactic McDonald cerclage, the anesthetized patient is placed in dorsal lithotomy position. At least one assistant is required to provide exposure using right angle or medium-sized Deaver retractors. While some surgeons prefer the use of a weighted speculum, we have found that individually placed retractors permit better exposure around the entire circumference, and especially for the posterior bites. After an antiseptic vaginal prep, the anterior ectocervix is grasped with a ring forceps, or similar non-traumatic instrument such as a Babcock clamp, used to provide counter traction. The urinary bladder is generally emptied prior to the procedure, although some surgeons recommend leaving some urine in the bladder to better define the position of the bladder as it reflects onto the ectocervix.
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