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Summary Table

Category	Details
Risk Factors	History of cervical procedures (conization, LEEP, mechanical dilation), prior second-trimester loss, uterine anomalies (bicornuate uterus, septate uterus), connective tissue disorders (Ehlers-Danlos syndrome), in utero DES exposure, history of cervical lacerations during delivery
Etiology	Structural weakness of the cervical stroma due to decreased collagen and smooth muscle integrity, leading to inability to maintain a closed cervix under the increasing gravitational load of the growing uterus
Patient Presentation	Painless cervical dilation in the second trimester (typically 16 to 24 weeks), often presenting with a sensation of pelvic pressure, increased vaginal discharge, or the sudden finding of membranes bulging through the os without preceding contractions
Classic Physical Exam	Painless dilation of the cervix on digital or speculum exam, visible or palpable membranes at or beyond the external os, absence of regular uterine contractions
Key Diagnostic Results	Transvaginal ultrasound showing short cervical length (less than 25 mm before 24 weeks), funneling of the internal os; the diagnosis is primarily clinical and historical
Management	History-indicated cerclage (McDonald or Shirodkar) placed at 12 to 14 weeks in patients with a classic history; ultrasound-indicated cerclage for short cervix with prior preterm birth; vaginal progesterone for short cervix without prior history of cervical insufficiency
Keywords	"Painless dilation," "second-trimester loss," "bulging membranes," "short cervix on ultrasound," "no contractions," "history of cone biopsy/LEEP"

1. Pathophysiology

The cervix normally functions as a mechanical barrier that remains closed throughout pregnancy, held together by a dense extracellular matrix rich in type I and type III collagen, along with smooth muscle and proteoglycans. In cervical insufficiency, this structural framework is compromised. The collagen content is either congenitally reduced (as in connective tissue disorders) or acquired through surgical trauma to the cervical stroma.

As the uterus enlarges during the second trimester, the gravitational force of the amniotic sac and fetus exerts downward pressure on the internal cervical os. In a normal cervix, this load is distributed across intact collagen fibers. In an insufficient cervix, the weakened stroma cannot resist this force, and the internal os begins to dilate passively. This is the key concept: the dilation is passive and mechanical, not driven by myometrial contractions. That is why the process is painless and why patients often present late, sometimes only when the membranes are already visible at the introitus.

The funneling seen on ultrasound represents the early phase of this process: the internal os opens first while the external os remains closed, creating a V-shaped or U-shaped indentation of membranes into the cervical canal. Progressive shortening follows as the entire canal gives way.

Once the membranes prolapse through the cervix, they become exposed to vaginal flora, dramatically increasing the risk of chorioamnionitis and subsequent preterm premature rupture of membranes (PPROM). This inflammatory cascade can then trigger contractions, converting a previously painless process into preterm labor. Understanding this sequence is important: infection is a consequence of cervical insufficiency, not its cause.

2. Classification and Clinical Manifestation

Classification	Basis	Clinical Scenario
History-indicated (classic)	Diagnosed based on a history of one or more painless second-trimester losses or deliveries without labor	Patient with two prior mid-trimester losses, each occurring without contractions, now presenting in early pregnancy requesting intervention
Ultrasound-indicated	Identified by cervical shortening on serial transvaginal ultrasound surveillance in a patient with risk factors	Patient with one prior spontaneous preterm birth found to have cervical length of 20 mm at 20 weeks on routine surveillance
Physical exam-indicated (rescue/emergent)	Found incidentally on exam with advanced cervical dilation and/or visible membranes, without contractions	Patient presenting at 22 weeks with pelvic pressure; speculum exam reveals membranes bulging through a 3 cm dilated cervix, no contractions on tocometry

Clinical Features

Feature	Detail
Timing	Almost always between 16 and 24 weeks of gestation
Pain	Absent or minimal; patients may report only vague pelvic heaviness
Vaginal discharge	Increased watery or mucoid discharge due to loss of the mucus plug
Contractions	Absent on tocodynamometry at presentation (this is the hallmark distinguishing feature)
Recurrence	Without intervention, the recurrence risk in subsequent pregnancies is approximately 30%

3. Diagnostic Workup

Test	Role	Key Findings
Transvaginal ultrasound	Best initial test for cervical assessment	Cervical length less than 25 mm, funneling of internal os
Speculum examination	Essential to evaluate for visible dilation, prolapsing membranes, and to rule out PPROM	Painless dilation, membranes at or beyond external os
Tocodynamometry	Used to exclude preterm labor as the cause of dilation	Absence of regular contractions supports the diagnosis
Fetal fibronectin (fFN)	Adjunctive; its primary value is its negative predictive value	A negative result helps exclude imminent delivery, but it does not confirm cervical insufficiency
Cultures / GBS screening	Performed when membranes are exposed or infection is suspected	Rule out chorioamnionitis before any surgical intervention

Cervical insufficiency is fundamentally a clinical and historical diagnosis. There is no single laboratory test that confirms it. The diagnosis is most confidently made retrospectively, after a patient has experienced one or more painless second-trimester losses without evidence of labor, abruption, or infection.

In practice, the workup begins with a transvaginal ultrasound to measure cervical length. This is the best initial screening tool. A cervical length of less than 25 mm before 24 weeks is the widely accepted threshold that triggers intervention. Funneling, defined as protrusion of the amniotic membranes into the internal os creating a funnel shape, adds supporting evidence but alone is not diagnostic.

The distinction from preterm labor is critical. In cervical insufficiency, tocodynamometry shows no regular uterine contractions. If the patient has cervical dilation with contractions, the diagnosis shifts to preterm labor, and the management pathway changes entirely (tocolysis, antenatal corticosteroids). This single finding on the toco strip is the most testable discriminator in this workup.

Speculum examination should be performed before any digital exam to avoid inadvertent rupture of membranes if they are prolapsing. If there is any suspicion of rupture, a sterile speculum exam with pooling, ferning, and nitrazine testing (or a commercially available PPROM assay) should be performed before proceeding.

4. Management and Treatment

Clinical Scenario	Intervention	Timing/Details
History-indicated cerclage	McDonald cerclage (purse-string suture around the cervix) or Shirodkar cerclage (suture placed at the level of the internal os after bladder dissection)	Placed at 12 to 14 weeks gestation, removed at 36 to 37 weeks
Ultrasound-indicated cerclage	McDonald cerclage	Placed when cervical length falls below 25 mm on serial surveillance, before 24 weeks, only in patients with a prior spontaneous preterm birth
Short cervix WITHOUT prior preterm birth	Vaginal progesterone (200 mg micronized progesterone suppository nightly)	Started when cervical length is less than 20 mm on mid-trimester ultrasound, continued until 36 weeks
Rescue (emergent) cerclage	McDonald cerclage with possible amniocentesis to rule out infection prior to placement	Considered in patients with advanced dilation (up to 4 cm) and visible membranes, only if no signs of infection, active bleeding, labor, or PPROM
Transabdominal cerclage	Placed via laparotomy or laparoscopy	Reserved for patients with failed prior transvaginal cerclage or those with very short or surgically absent cervix; ideally placed preconceptionally or in early first trimester

History-indicated cerclage is the most straightforward management decision. A patient who presents with a classic history of two or more painless second-trimester losses should receive a cerclage at 12 to 14 weeks without waiting for ultrasound evidence of shortening. The McDonald technique is most commonly tested: a nonabsorbable suture (Mersilene tape or Prolene) is placed in a purse-string fashion around the cervix at the cervicovaginal junction. It is removed electively at 36 to 37 weeks or earlier if labor begins.

Ultrasound-indicated cerclage requires both a short cervix (less than 25 mm) and a history of prior spontaneous preterm birth before 34 weeks. This combination is essential. A short cervix alone, without a relevant obstetric history, is managed with vaginal progesterone, not cerclage. This is one of the most commonly tested distinctions.

For patients with a short cervix found incidentally on anatomy scan (around 18 to 22 weeks) and no prior preterm birth history, the standard of care is vaginal progesterone 200 mg nightly. This has been shown to reduce the rate of preterm birth in this population. An alternative is the Arabin pessary, though evidence for its efficacy is less consistent.

Rescue cerclage is a high-risk intervention. Before placing it, amniocentesis should be considered to rule out subclinical intra-amniotic infection (elevated IL-6 or positive culture). Contraindications to rescue cerclage include active contractions, clinical chorioamnionitis (maternal fever, uterine tenderness, fetal tachycardia), PPROM, and vaginal bleeding. If any of these are present, the cerclage should not be placed.

Activity restriction and pelvic rest are commonly recommended after cerclage placement, though strict bed rest has not been shown to improve outcomes and is no longer routinely advised.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Preterm labor	Also presents with cervical dilation in the second or early third trimester	Preterm labor has regular uterine contractions on tocometry; cervical insufficiency is painless with no contractions
Preterm premature rupture of membranes (PPROM)	Can present with watery vaginal discharge and subsequent cervical changes	PPROM presents with a gush of fluid, positive pooling/ferning/nitrazine; in cervical insufficiency, membranes are intact but prolapsing
Placental abruption	Can cause second-trimester pregnancy loss	Abruption presents with painful vaginal bleeding, uterine tenderness, and often fetal distress; cervical insufficiency is painless
Chorioamnionitis	Can lead to cervical dilation and second-trimester loss	Chorioamnionitis presents with maternal fever, uterine tenderness, fetal tachycardia, and often follows prolonged rupture of membranes; it can be a complication of cervical insufficiency, not the primary event
Normal cervical shortening of late pregnancy	Cervical length decreases physiologically in the third trimester	Physiologic shortening occurs after 28 to 32 weeks; cervical insufficiency presents much earlier, typically before 24 weeks
Mullerian anomaly with pregnancy loss	Patients with uterine anomalies may have recurrent second-trimester losses	Mullerian anomalies cause loss due to inadequate uterine cavity volume or abnormal placentation, not painless cervical dilation; however, some anomalies coexist with true cervical insufficiency

6. Traps and High-Yield Pearls

The single most common trap with cervical insufficiency questions involves confusing it with preterm labor. Both can present with cervical dilation in the second trimester, and a rushed reader may jump to tocolysis. The vignette will deliberately emphasize the absence of contractions and the painless nature of the dilation. If you see "painless cervical dilation at 20 weeks with no contractions," the answer is cervical insufficiency, not preterm labor.

The second major trap tests whether you know the indications for cerclage versus vaginal progesterone. A short cervix on ultrasound does not automatically mean cerclage. Without a prior preterm birth history, the correct answer is vaginal progesterone. Cerclage in that population has not shown benefit and is not recommended. Test-writers love presenting a primigravida with a short cervix and offering cerclage as an answer choice to catch students who do not understand this distinction.

Another high-yield pearl involves the rescue cerclage scenario. The vignette will describe a patient with bulging membranes and dilated cervix, then ask about the next best step. Before offering cerclage, you must rule out infection and confirm intact membranes. If the question includes any fever, elevated WBC with left shift, or uterine tenderness, the answer shifts to delivery or expectant management with antibiotics, not cerclage.

Finally, remember the gestational age boundaries. Cerclage is generally not placed after 24 weeks because the risks of the procedure outweigh the benefits at that point, and viability-based management (antenatal corticosteroids, magnesium for neuroprotection) takes over. If a question describes a patient at 26 weeks with a short cervix, the answer is not cerclage.

The core competency being tested is pattern recognition of the painless, contractionless second-trimester dilation, followed by correct stratification of management based on the patient's obstetric history and gestational age.

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