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Obstetric Imaging
Demonstration data

Cervical Length & Preterm Risk

Stratifies mid-trimester spontaneous preterm birth risk from transvaginal cervical length, accounting for singleton versus multiple gestation.

Transvaginal cervical length measured in the mid-trimester (typically 16–24 weeks) is inversely related to the risk of spontaneous preterm birth. A short cervix identifies candidates for progesterone and, in selected cases, cerclage or a pessary. The measurement is more reproducible and predictive than transabdominal assessment.

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📐 Formula

Mid-trimester TVUS cervical length: <25 mm increased risk; <15 mm high risk

When to Use

  • Mid-trimester screening for spontaneous preterm birth, especially with a prior preterm birth.
  • Evaluation of symptoms suggesting cervical shortening or insufficiency.
  • Guiding progesterone, cerclage or pessary decisions.

How It Works

  • Perform transvaginal ultrasound with an empty maternal bladder and no undue probe pressure.
  • Measure the closed cervical canal from internal to external os in a sagittal plane; take the shortest of three measurements.
  • Assess for funnelling of the internal os and dynamic shortening.

Interpreting the Result

  • Singleton, cervical length <25 mm before 24 weeks: increased risk of spontaneous preterm birth — consider vaginal progesterone.
  • Cervical length <15 mm: high risk, with a substantial proportion delivering preterm.
  • In twins a short cervix is also predictive, though management thresholds and interventions differ from singletons.

Limitations & Pitfalls

  • A single measurement is a snapshot; dynamic change and funnelling add information.
  • Predictive thresholds are gestational-age and plurality specific; do not extrapolate singleton cut-offs directly to multiples.
  • Technique errors (excess probe pressure, full bladder) falsely lengthen the cervix.

References

  • Iams JD, Goldenberg RL, Meis PJ, et al. The length of the cervix and the risk of spontaneous premature delivery. N Engl J Med. 1996;334(9):567-572.
  • Society for Maternal-Fetal Medicine. The role of cervical length screening. Am J Obstet Gynecol. 2016;215(3):B2-B7.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.