At what gestational age should transvaginal cervical‑length screening be initiated in a woman with a history of spontaneous preterm labor or delivery?

Cervical Length Screening in Women with Prior Preterm Birth

Begin transvaginal cervical length screening at 16 weeks of gestation in women with a history of spontaneous preterm birth, and continue serial measurements every 1–2 weeks through 23 6/7 weeks of gestation. 1, 2

Screening Protocol and Timing

• Start screening at 16 weeks and perform serial transvaginal ultrasound measurements approximately every 2 weeks until 23 6/7 weeks of gestation in singleton pregnancies with prior spontaneous preterm birth. 1, 2

• The screening window extends through 24 weeks of gestation (specifically 16 0/7 to 23 6/7 weeks), as this is when cervical shortening is most predictive and interventions remain effective. 1, 2

• Transvaginal ultrasound is mandatory—transabdominal measurements are insufficient and unreliable for clinical decision-making. 3, 1

• Approximately 40% of women with prior spontaneous preterm birth will develop a short cervix (<25 mm) during serial screening, identifying those who benefit most from intervention. 1

Critical Measurement Thresholds and Actions

For Cervical Length <25 mm Before 24 Weeks

• Offer ultrasound-indicated cerclage when cervical length shortens to <25 mm, as this intervention reduces preterm birth <35 weeks by 30% and composite perinatal morbidity/mortality by 36%. 1

• If cervical length is <10 mm, cerclage shows particular benefit with decreased preterm birth at <35 weeks (39.5% vs 58.0%). 4

• Continue vaginal progesterone 200 mg daily if already started, or initiate it at the time of cerclage placement and maintain through 36–37 weeks. 4

For Cervical Length ≤20 mm Before 24 Weeks

• Prescribe vaginal progesterone 200 mg daily (GRADE 1A recommendation) if cerclage is declined or not indicated, as this reduces preterm birth by approximately 45% with demonstrated neonatal benefits. 3, 1

For Cervical Length 21–25 mm Before 24 Weeks

• Offer vaginal progesterone through shared decision-making (GRADE 1B recommendation), discussing the moderate evidence for benefit in this range. 3

Subtype-Specific Considerations

• Women with prior preterm birth due to preterm labor with intact membranes have shorter cervical lengths (mean 3.28 cm at 24–30 weeks) compared to those with prior preterm premature rupture of membranes (mean 3.77 cm). 5

• Both subtypes carry increased risk of recurrent spontaneous preterm birth, and cervical length <3.0 cm predicts preterm birth <35 weeks with 63.6% sensitivity and 93.8% negative predictive value. 5

• The screening and management protocol applies equally to both subtypes of prior spontaneous preterm birth. 5

Monitoring After Intervention

• Approximately 69% of high-risk women maintain cervical length >25 mm throughout pregnancy and do not require cerclage, underscoring the value of serial screening over prophylactic intervention. 4

• If cerclage is placed, continue vaginal progesterone 200 mg daily as adjunctive therapy, which reduces spontaneous preterm birth <34 weeks (2.2% vs 18.4%) and <37 weeks (9.1% vs 29.7%). 4

• Do not perform routine serial ultrasound after cerclage placement, as evidence does not support clinical benefit from continued monitoring, though shorter cervical length below the cerclage correlates with higher preterm birth risk. 4

Interventions to Avoid

• Do NOT use 17-alpha hydroxyprogesterone caproate (17-OHPC) for short cervix treatment in women with prior preterm birth (GRADE 1B recommendation against). 3, 4

• Do NOT place cervical pessary due to conflicting trial data and recent safety signals including increased perinatal mortality (GRADE 1B recommendation against). 3, 4

• Do NOT prescribe bed rest, as it offers no benefit and may be harmful; permit routine daily activities and light upper-body exercise while avoiding moderate-to-vigorous physical activity. 4

Common Pitfalls to Avoid

• Avoid placing cerclage at arbitrary gestational ages without objective cervical shortening documented by transvaginal ultrasound—prophylactic cerclage should be reserved only for women with ≥3 prior second-trimester losses. 4

• Do not rely on clinical cervical examination alone—transvaginal ultrasound provides superior risk stratification and is the reference standard. 4

• Do not screen after 24 weeks of gestation, as the predictive value diminishes and proven effective interventions are no longer available. 6

• Ensure proper transvaginal ultrasound technique with continuing quality assurance—measurements must follow standardized protocols (e.g., Perinatal Quality Foundation or Fetal Medicine Foundation guidelines). 3, 1