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Cervical length (CL) measurement by transvaginal ultrasound (TVU) at 18–24 weeks of gestation is the most reliable predictor of spontaneous preterm birth currently available in clinical practice. The cervix normally measures 35–48 mm in mid-pregnancy and progressively shortens as term approaches. A transvaginal cervical length below 25 mm at 20–24 weeks in a singleton pregnancy is the internationally accepted threshold for identifying women at high risk of spontaneous preterm birth (sPTB) before 34 weeks. In women with a previous spontaneous preterm birth and a current short cervix, the risk of recurrent preterm birth before 34 weeks is approximately 30–50%, compared to a background risk of 1–2% in low-risk populations. The primary evidence-based interventions for short cervical length are: (1) vaginal progesterone (200–400 mg micronised progesterone pessary nightly), which reduces the risk of sPTB by approximately 45% in women with a short cervix identified by TVU screening; (2) cervical cerclage (a surgical suture encircling the cervix), which reduces preterm birth risk by approximately 25% in women with both a short cervix (<25 mm) and a prior preterm birth or cervical insufficiency history; and (3) cervical pessary (an Arabin pessary), which has shown benefit in some populations, particularly in women with twins and a short cervix, though evidence remains mixed. Fetal fibronectin (fFN) is a biochemical test that, when negative, effectively rules out preterm birth within 7–14 days and can guide decisions on hospitalisation and tocolysis. The combination of TVU cervical length and fFN has higher predictive value than either test alone.
Preterm birth risk: TVU CL <25 mm at 20-24 weeks = high risk; CL <15 mm = very high risk. Progesterone 200 mg vaginally at night; Cerclage if CL <25 mm + prior PTB history
- 1Perform transvaginal ultrasound (TVU) with the woman in the supine or dorsal lithotomy position with a comfortably full (not empty) bladder; empty bladder before insertion of the probe as overfilling falsely elongates the cervix.
- 2Insert the probe gently into the anterior fornix; obtain a sagittal view of the cervix showing the internal os, the cervical canal, and the external os clearly.
- 3Measure the functional cervical length from the internal os to the external os along the cervical canal in a straight line (or along the curve for a curved canal); take three measurements and record the shortest.
- 4Note any cervical funnelling (dilatation of the internal os into a V or U shape); the functional length is measured from the tip of the funnel to the external os.
- 5Classify risk: CL ≥25 mm = lower risk (continue routine care); CL 20–24 mm = intermediate (consider progesterone); CL <20 mm = high risk (progesterone + consider cerclage); CL <10 mm = very high risk (emergency cerclage or pessary may be indicated).
- 6Test fetal fibronectin (fFN) from a vaginal swab in symptomatic women; a negative fFN (<50 ng/mL) has a negative predictive value of 96% for preterm birth within 7 days.
- 7For women with CL <25 mm: initiate vaginal progesterone 200–400 mg nightly; refer to specialist obstetric clinic; monitor every 1–2 weeks. If CL <25 mm and history of prior PTB, discuss cervical cerclage with a maternal-fetal medicine specialist.
OPPTIMUM trial and Fonseca trial support progesterone for asymptomatic short cervix in singletons
A CL of 18 mm is significantly below the 25 mm threshold. Progesterone reduces the risk of preterm birth before 34 weeks by approximately 45%. No cerclage is indicated without a prior preterm birth history in nulliparous women.
CERCLAGE trial and Owen et al NEJM 2009 support cerclage in this scenario; risk reduction approximately 25%
The combination of prior spontaneous PTB and CL <25 mm in the current pregnancy is the strongest indication for cerclage. Progesterone and cerclage together may be more effective than either alone.
Normal cervical length provides reassurance; repeat measurement not routinely needed in low-risk women
A CL of 38 mm at 24 weeks is well within the normal range. The risk of spontaneous preterm birth before 34 weeks in this woman is approximately 1–2%.
Evidence for cerclage in twins without prior PTB is limited; pessary shows benefit in some trials but evidence is mixed
Short cervix in twin pregnancy carries higher absolute risk of preterm birth than in singletons. Arabin pessary placement at CL <25 mm in twins has shown benefit in some but not all RCTs. Progesterone 200 mg vaginally is widely offered.
Universal TVU cervical length screening at the 18–22 week anomaly scan to identify asymptomatic women at risk.. This application is commonly used by professionals who need precise quantitative analysis to support decision-making, budgeting, and strategic planning in their respective fields
Guiding decisions on progesterone therapy and cerclage in women with a short cervix.. Industry practitioners rely on this calculation to benchmark performance, compare alternatives, and ensure compliance with established standards and regulatory requirements
Fetal fibronectin testing in symptomatic women with threatened preterm labour to guide hospitalisation and steroid administration.. Academic researchers and students use this computation to validate theoretical models, complete coursework assignments, and develop deeper understanding of the underlying mathematical principles
Monitoring cervical length in women with risk factors (prior PTB, cone biopsy, uterine anomaly) through serial measurements.. Financial analysts and planners incorporate this calculation into their workflow to produce accurate forecasts, evaluate risk scenarios, and present data-driven recommendations to stakeholders
Research studies evaluating novel interventions (cervical pessary, combination therapies) for preterm birth prevention.. This application is commonly used by professionals who need precise quantitative analysis to support decision-making, budgeting, and strategic planning in their respective fields
Women with uterine anomalies
{'title': 'Women with uterine anomalies', 'body': 'Uterine anomalies (bicornuate, septate, or unicornuate uterus) increase preterm birth risk independently of cervical length. TVU measurement is still recommended but may be technically more difficult. Surgical correction of a uterine septum (hysteroscopic metroplasty) before conception is associated with improved preterm birth outcomes.'} When encountering this scenario in cervical length risk calculations, users should verify that their input values fall within the expected range for the formula to produce meaningful results. Out-of-range inputs can lead to mathematically valid but practically meaningless outputs that do not reflect real-world conditions.
Emergency cerclage (rescue cerclage)
{'title': 'Emergency cerclage (rescue cerclage)', 'body': 'Emergency (rescue) cerclage may be offered when cervical dilatation is noted on clinical examination before 24 weeks in the absence of active labour or infection, particularly with prolapsed membranes visible at the external os. Evidence is limited, but emergency cerclage can extend pregnancy by a median of 4–6 weeks in selected cases.'}
Preterm PROM (premature rupture of membranes)
{'title': 'Preterm PROM (premature rupture of membranes)', 'body': 'Cervical length measurement is of limited value once membranes have ruptured, as the fluid leakage and potential infection change the clinical management entirely. Cerclage is contraindicated after PROM due to infection risk. Management shifts to monitoring for infection, antenatal corticosteroids, and timing of delivery.'}
Cerclage removal and timing
{'title': 'Cerclage removal and timing', 'body': 'Elective cerclage is typically removed at 36–37 weeks to allow spontaneous labour onset. If a woman presents in spontaneous preterm labour with an intact cerclage, the suture should be removed urgently (if gestation warrants) to prevent cervical laceration during delivery. Cerclage can be left in place if delivery is being deferred with tocolysis.'}
| CL at 20-24 weeks | Risk Level | Recommended Action |
|---|---|---|
| ≥25 mm | Low risk | Routine care; no intervention |
| 20–24 mm | Intermediate | Vaginal progesterone 200 mg nightly; specialist referral |
| 10–19 mm | High risk | Progesterone; cerclage if prior PTB; fortnightly monitoring |
| <10 mm | Very high risk | Emergency specialist review; cerclage or pessary; corticosteroids for fetal lung maturity if viable |
Why must transvaginal ultrasound be used rather than transabdominal?
Transabdominal ultrasound significantly overestimates cervical length, especially when the bladder is full. Transvaginal ultrasound (TVU) provides a direct, unobstructed view of the cervix and gives reproducible measurements. TVU is safe in pregnancy, including in women with low-lying placentae, and is the only accepted method for clinical cervical length measurement. This matters because accurate cervical length risk calculations directly affect decision-making in professional and personal contexts.
Should all pregnant women be screened for cervical length?
Universal TVU cervical length screening at the 20-week anomaly scan has been adopted in several countries (UK, US, Israel) as it allows identification of women with an incidentally short cervix who would otherwise have no identified risk factor. The Society for Maternal-Fetal Medicine (SMFM) recommends universal screening in singleton pregnancies. Some guidelines recommend targeted screening in higher-risk women only.
What is cervical funnelling and does it matter?
Cervical funnelling (also called beaking) refers to dilatation of the internal os that extends into the cervical canal, creating a V or U shape on ultrasound. The functional cervical length is measured from the funnel tip to the external os. Funnelling with a short functional cervical length increases preterm birth risk. Funnelling alone without a short functional length is not independently predictive.
How does vaginal progesterone reduce preterm birth?
The exact mechanism is not fully understood, but progesterone suppresses myometrial contractility by downregulating gap junction expression and oxytocin receptor sensitivity. It may also reduce local cervical inflammation (implicated in preterm cervical remodelling). Vaginal administration provides high local concentrations with lower systemic levels compared to oral progesterone. The process involves applying the underlying formula systematically to the given inputs. Each variable in the calculation contributes to the final result, and understanding their individual roles helps ensure accurate application.
Is there a difference between cervical cerclage types?
The most common techniques are McDonald cerclage (purse-string suture at the cervicovaginal junction) and Shirodkar cerclage (suture placed higher, closer to the internal os, with anterior and posterior mucosal dissection). For women who have failed two previous cervical cerclages, abdominal cerclage (laparoscopic or open) placed at the level of the internal os before or during pregnancy offers the highest stitch placement and lowest failure rate.
What is fetal fibronectin (fFN) and when is it useful?
Fetal fibronectin is a protein found in the chorioamniotic membranes that acts as a biological glue. It is normally absent from cervical-vaginal secretions between 22 and 34 weeks. Its presence above 50 ng/mL suggests disruption of the maternal-fetal interface and predicts increased preterm birth risk. Crucially, a negative fFN result (below 50 ng/mL) effectively rules out delivery within the next 7–14 days with >99% NPV, avoiding unnecessary hospitalisation.
What should women with a short cervix avoid?
Women with a short cervix are often advised to avoid heavy lifting, vigorous exercise, and sexual intercourse (specifically orgasm, which causes uterine contractions) until further evaluation and treatment are in place. However, evidence for activity restriction reducing preterm birth is limited. The primary intervention is vaginal progesterone and/or cerclage, not bed rest.
Can a cervix that was short become longer?
Cervical length can appear to lengthen on serial measurement in some women, though true anatomical change is unusual. Apparent lengthening is often due to measurement variability, resolution of cervical oedema, or the effect of progesterone reducing inflammatory cervical remodelling. Clinical decisions should be based on the shortest reproducible measurement. This is an important consideration when working with cervical length risk calculations in practical applications.
Pro Tip
Always perform TVU cervical length measurement before digital cervical examination at the 20-week scan visit, as a digital exam can artificially stimulate fFN release and disturb amniotic membranes. The TVU probe should be introduced gently just to the anterior fornix, not deep into the cervical os.
Did you know?
Before TVU cervical length screening became standard, cervical incompetence (the old term for cervical insufficiency) was almost entirely diagnosed retrospectively — after a painless second-trimester miscarriage or very early preterm birth. TVU screening transformed this from a diagnosis of exclusion made after disaster into a preventable condition identified proactively.
References
- ›Romero R et al — Vaginal progesterone in women with an asymptomatic sonographic short cervix — Am J Obstet Gynecol 2012
- ›Owen J et al — Midtrimester cervical length and the risk of preterm delivery — NEJM 2009
- ›NICE — Preterm Labour and Birth (NG25)
- ›SMFM — Progesterone and Preterm Birth Prevention (Practice Bulletin)
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