Cervical Length and Risk of Preterm Birth Calculator -BETA
Purpose: This calculator estimates spontaneous preterm birth risk using published cervical-length models. The default singleton pathway uses the Celik/To/Nicolaides Fetal Medicine Foundation model with cervical length and obstetric history. A separate Berghella table-based pathway is available for high-risk singleton patients and can be actively selected from the evidence-pathway menu, and a BMJ Medicine individual-participant-data meta-analysis pathway is used for twin pregnancy.
Scope: Intended for asymptomatic patients with intact membranes and no cervical dilation or visible membranes. If contractions, bleeding, suspected rupture of membranes, cervical dilation, or visible membranes are present, the calculator will display a triage warning rather than a reassuring numerical risk estimate.
Patient and ultrasound inputs
weeks
days
Use the earliest prior spontaneous PTB, PPROM, or mid-trimester spontaneous delivery/loss when known.
Use only for asymptomatic patients with intact membranes and no cervical dilation or visible membranes. Clinical evaluation supersedes calculator output.
weeks
days
Allowed gestational-age window will update by evidence pathway.
mm; use shortest technically adequate closed cervical length
Before calculating: This estimate applies to asymptomatic patients with intact membranes and no cervical dilation. Do not use as a substitute for triage when contractions, bleeding, suspected ROM, visible membranes, or cervical dilation are present.
Risk estimate and counseling output
Enter patient information and select Calculate.
Clinical interpretation and model applicability
| Scenario | Use / interpretation |
|---|---|
| Celik/FM default singleton pathway | Restricted in this calculator to asymptomatic singleton pregnancies at 20+0 to 24+6 weeks, matching the Celik/FM study window used for defensible application. It combines transvaginal cervical length with obstetric history: nulliparous, previous term birth, previous delivery/loss at 16–23 weeks, previous spontaneous PTB at 24–33 weeks, or previous spontaneous PTB at 34–36 weeks. |
| High-risk singleton pathway: explicit definition | Use the Berghella table pathway when the patient has a prior spontaneous PTB or spontaneous mid-trimester delivery/loss, prior cervical excisional surgery such as cone biopsy or LEEP, known Müllerian anomaly, or another established clinician-entered high-risk factor for spontaneous PTB/cervical insufficiency. It is table-based by CL and gestational age at measurement; entry is restricted to 15+0 to 28+6 weeks for application to the Berghella study range. |
| Singleton, no prior spontaneous PTB | Short cervix is commonly defined as TVUS cervical length ≤25 mm in the mid-trimester. SMFM #70 recommends vaginal progesterone at ≤20 mm before 24 weeks and consideration at 21–25 mm using shared decision-making. |
| Singleton, no prior spontaneous PTB, CL 10–25 mm, no dilation | SMFM #70 recommends against cerclage in the absence of cervical dilation and recommends against pessary for singleton short cervix. |
| Twin pregnancy | Restricted in this calculator to 16+0 to 26+6 weeks for the Hughes twin IPDMA pathway. The BMJ Medicine twin IPDMA found risk is continuous; no single cervical length cutoff reliably identifies all high- or low-risk twin pregnancies. SMFM #70 recommends against routine progesterone, pessary, or cerclage for twin cervical shortening outside a clinical trial. |
| Symptoms, dilation, visible membranes, or ROM concern | This calculator is not an acute triage tool. If symptoms, suspected membrane rupture, cervical dilation, or visible membranes are selected, the calculator displays a triage warning rather than a potentially misleading numerical estimate. |
Arrange urgent obstetric triage when symptoms, suspected rupture of membranes, bleeding, cervical dilation, visible membranes, or a very short cervix are present. The calculator is not intended to replace acute evaluation.
| Finding | Suggested action |
|---|---|
| Symptoms, bleeding, ROM concern, dilation, or visible membranes | Do not rely on a screening-risk estimate. Send for same-day clinical evaluation/triage and involve MFM based on gestational age, exam findings, and local resources. |
| Singleton, no prior spontaneous PTB, TVUS CL ≤25 mm before 24 weeks | Consider MFM referral or consultation, especially if CL ≤20 mm, progressive shortening, funneling/sludge, uncertainty about measurement, or questions about progesterone versus additional evaluation. |
| Prior spontaneous PTB or mid-trimester loss and TVUS CL ≤25 mm before 24 weeks | Refer to MFM for individualized recurrent-preterm-birth management and assessment for ultrasound-indicated cerclage or other interventions. |
| Prior cerclage, suspected cervical insufficiency, prior cone/LEEP with short cervix, Müllerian anomaly, or recurrent second-trimester losses | MFM consultation is appropriate even if the calculated risk appears modest, because treatment decisions depend on obstetric history and cervical examination. |
| Twin pregnancy with cervical shortening, especially CL ≤25 mm or rapidly shortening cervix | Refer to MFM. Routine progesterone, pessary, or cerclage for twin cervical shortening is not recommended outside a clinical trial, but individualized counseling and surveillance are appropriate. |
| CL <10 mm at any applicable gestational age | Same-day MFM/obstetric evaluation is reasonable to assess for dilation or exposed membranes and to determine gestational-age-specific management. |
Patient handout: Short Cervix in Pregnancy.
Cervical length measurement quality
- Use transvaginal ultrasound for measurements that guide therapy.
- Patient bladder should be empty; avoid excessive probe pressure that can artificially lengthen the cervix.
- Record the shortest technically adequate closed cervical length after observing long enough to assess dynamic change/funneling.
- Transabdominal or transperineal measurements may be useful as screening approaches, but therapeutic decisions should be based on standardized TVUS measurement.
Model details
Celik/To/Nicolaides FMF singleton model
Uses published likelihood ratios for cervical length and obstetric history. Output should be labeled as an FMF/Celik population-calibrated estimate rather than a definitive individualized probability. The model reports interval-specific risks for spontaneous delivery <28 weeks, 28–30 weeks, 31–33 weeks, and 34–36 weeks. Baseline interval risks are the study cohort rates: 0.23%, 0.24%, 0.57%, and 2.93%.
post-test odds = baseline odds × CL likelihood ratio × obstetric-history likelihood ratio
Berghella high-risk singleton pathway
Uses table-based predicted probabilities by cervical length and gestational age at measurement, with bilinear interpolation between table cells. The <35 and <32-week outcomes are restricted to GA 15+0 to 28+6. The <28-week outcome is restricted to GA 15+0 to 25+6 and is not calculated outside that range.
BMJ Medicine twin IPDMA pathway
Uses baseline rates from the twin IPDMA and hazard ratios per additional millimeter of cervical length: HR 0.960 for sPTB <37 weeks and HR 0.932 for sPTB <34 weeks. Results are approximate counseling estimates because the source model is a time-to-event meta-analysis rather than a simple absolute-risk table.
Foundational evidence note
Iams et al. established the continuous relationship between shorter mid-trimester transvaginal cervical length and increased spontaneous preterm delivery risk. This calculator uses later risk models for calculation, but Iams remains an important foundational reference for the measurement approach and risk continuum.
Patient education and references
Patient handout: Short Cervix in Pregnancy.
- Celik E, To M, Gajewska K, Smith GCS, Nicolaides KH. Cervical length and obstetric history predict spontaneous preterm birth: development and validation of a model to provide individualized risk assessment. Ultrasound Obstet Gynecol. 2008;31:549-554.
- Berghella V, Roman A, Daskalakis C, Ness A, Baxter JK. Gestational age at cervical length measurement and incidence of preterm birth. Obstet Gynecol. 2007;110:311-317.
- Hughes KM, Aberoumand M, Seidler AL, et al. Prognostic value of cervical length for spontaneous preterm birth in asymptomatic women with twin pregnancy: meta-analysis of individual participant data. BMJ Medicine. 2025;4:e000877.
- 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;334:567-572.
- Society for Maternal-Fetal Medicine Consult Series #70: Management of short cervix in individuals without a history of spontaneous preterm birth. Am J Obstet Gynecol. 2024.