Cervical Length Calculator — Preterm Birth Risk
The cervical length assessment evaluates the risk of preterm birth based on transvaginal ultrasound measurement of cervical length. Short cervix (<25 mm) is a strong predictor of spontaneous preterm birth, and early detection enables timely interventions such as progesterone therapy, cervical cerclage, and enhanced fetal surveillance.
About
Cervical length is measured via transvaginal ultrasound from the internal to external cervical os. A length >30 mm is considered normal. Length of 25-30 mm is borderline and may warrant serial monitoring. Length <25 mm indicates a short cervix with significantly increased risk of preterm birth. The risk increases further as cervical length decreases, with lengths <15 mm associated with very high risk.
Formula
Cervical Length (mm) measured by transvaginal ultrasound; Risk Category determined by thresholds: Normal >30 mm, Borderline 25-30 mm, Short <25 mm, Very Short <15 mm
The cervical length is directly measured via transvaginal ultrasound. Risk stratification follows ACOG guidelines: >30 mm is low risk, 25-30 mm is borderline, <25 mm is short cervix, and <15 mm is very short. Additional risk factors include prior preterm birth, multiple gestation, and history of cervical insufficiency.
Score Interpretation
Preterm birth (<37w) affects 10% of pregnancies worldwide and is the leading cause of neonatal morbidity and mortality. Short cervix is one of the strongest predictors of spontaneous preterm birth. Transvaginal cervical length screening at 18-24 weeks in high-risk women, combined with appropriate interventions (progesterone, cerclage, pessary), reduces preterm birth rates by up to 45%.
Low Risk — 30.01+
Cervical length >30 mm — low risk of preterm birth.
Management: Continue routine antenatal care.
Borderline — 25–30
Cervical length 25-30 mm — borderline risk.
Management: Consider serial transvaginal ultrasound monitoring.
Short Cervix — 15–24.99
Cervical length <25 mm — increased risk of preterm birth.
Management: Consider vaginal progesterone; cerclage if prior PTB.
Very Short Cervix — 0–14.99
Cervical length <15 mm — very high risk of preterm birth.
Management: Urgent MFM referral; corticosteroids, MgSO4, cerclage if <24w.
Reference Ranges
| Population | Normal Range | Notes |
|---|---|---|
| Normal (singleton, no prior PTB) | >30 mm | Low risk of preterm birth |
| Borderline | 25-30 mm | Consider serial monitoring |
| Short cervix | <25 mm | Increased PTB risk; intervention indicated |
Dr. Khaled Hassan
Dr. Khaled is a cardiology consultant with experience in acute cardiac care.
View medical review board & editorial policy →Example Calculation
A 28-year-old G1P0 at 20w has cervical length 22 mm, no prior PTB. Result: Short cervix. Recommendation: Vaginal progesterone 200 mg daily, serial ultrasound monitoring.
Related Conditions
Common Mistakes
Measuring cervical length via transabdominal ultrasound
Transvaginal ultrasound is the gold standard for cervical length measurement as it provides better visualization and reproducibility.
Frequently Asked Questions
At what gestational age should cervical length be screened?
What interventions are proven for short cervix?
References
- ACOG Practice Bulletin No. 234: Prediction and Prevention of Spontaneous Preterm Birth. Obstet Gynecol. 2021;138(2):e57-e72.
- Fonseca EB et al. Progesterone and the risk of preterm birth among women with a short cervix. NEJM. 2007;357(5):462-469. PubMed