Mid-Pregnancy · Preterm Risk
Cervical Length Scan & Preterm Risk
Cervical length scan at 18-24 weeks measures the closed cervical canal. Normal >30 mm; short ≤25 mm = preterm birth risk. Progesterone reduces preterm birth by ~35% in short cervix pregnancies. Plus cerclage (stitch), what to watch for, and how to manage anxiety. SMFM Consult #70 / NICE NG25.
Last reviewed 2 June 2026
Cervical length — TV ultrasound 16-24 weeks
What does my cervical length mean?
mm
wk
Enter cervical length (mm) and gestational age (weeks) to see interpretation.
Educational tool only — not medical advice. CL measurement must be by TRANSVAGINAL ultrasound (transabdominal underestimates). Optimal window 16-24 weeks. SMFM Consult #70 (2023) supports universal mid-trimester CL screening with vaginal progesterone for ≤ 25 mm in singletons without prior PTB; ≤ 20 mm is the lower SMFM threshold.
What does this mean?
Cervical length on a transvaginal scan at 16–24 weeks is the single best predictor of spontaneous preterm birth. Iams 1996 (NEJM) showed risk rises as CL falls — at CL ≤ 25 mm at 22–24 wk, the risk of delivery before 35 weeks is about 6× higher than in women with normal CL. The good news: it’s treatable. The PREGNANT trial (Hassan 2011) and subsequent meta-analyses showed that vaginal progesterone 200 mg nightly for short cervix without prior PTB cuts the <33 wk delivery rate by ~45 %. For short cervix WITH a prior preterm birth, an cerclage (cervical stitch) + progesterone combination is offered (NICE NG25, ACOG PB 234). Transabdominal scan over-estimates CL — insist on the transvaginal (sterile, painless, takes ~2 minutes). Universal screening at the routine anomaly scan is now SMFM-recommended (Consult #70 2023).
What is a cervical length scan?
Transvaginal ultrasound measuring how long your cervix is — the closed canal between the womb and the vagina.
Done between 16 and 24 weeks (often at the 18-22 week anomaly scan). Not painful; takes a few minutes. Smaller scanning probe than usual; empty bladder first.
Normal vs short
- Normal: >30 mm at 18-24 weeks.
- Short: ≤25 mm — the single most powerful predictor of preterm birth.
Risk by length
- 25 mm: ~10% chance preterm.
- 15 mm: ~30%.
- 10 mm: ~50%.
- 5 mm: ~75%.
NOT guaranteed — many short cervix pregnancies reach term with intervention.
Who gets scanned?
- Universal screen at 18-22 weeks in some UK / US units (SMFM Consult #70 supports).
- High-risk: previous preterm birth, cervical surgery (LLETZ / cone), multiple pregnancy, uterine anomalies.
- NHS practice varies; private scans £150-300.
If your cervix is short
- Vaginal progesterone (200 mg pessary nightly) from 16-22 to 34-36 weeks — reduces preterm birth ~35% (Romero meta-analysis).
- Cervical cerclage (stitch) — surgical option in selected cases.
- Arabin pessary — mixed evidence.
- Bed rest NOT recommended — may harm.
Specialist review needed.
Cervical stitch (cerclage)
- History-indicated: placed at 12-14 wk based on previous late miscarriages / preterm births.
- Ultrasound-indicated: placed at 16-22 wk if short cervix + previous preterm.
- Rescue / emergency: placed if cervix already dilated.
Spinal/regional anaesthetic. Removed at 36-37 weeks (or in labour). Risks: infection, membrane rupture (~1-5%).
Will I deliver early?
Not necessarily. With treatment, most short cervix singleton pregnancies reach term or near-term.
- Without treatment: ~30-50% deliver before 35 wk.
- With progesterone: ~20% before 35 wk.
- Cerclage further reduces in selected cases.
Each week gained reduces complications. Even if preterm, 32-34 weeks has excellent outcomes.
Is it my fault?
No. Causes: genetic/anatomical; previous cervical surgery (LLETZ, cone biopsy); multiple pregnancy; uterine anomalies; prior preterm; infection sometimes; often idiopathic.
NOT caused by exercise, sex, stress, foods, working.
Signs of preterm labour to watch for
- Regular contractions (every 10 min or more), even painless.
- Vaginal bleeding.
- Waters breaking (gush or trickle).
- Pressure / heaviness in pelvis, backache.
- Unusual discharge (mucus plug).
- Reduced fetal movements.
Any of these: go to maternity unit same day. Early treatment can delay birth 24-48h to allow steroids for baby’s lungs.
Antenatal steroids
Betamethasone or dexamethasone given 24-34 weeks if preterm birth threatening:
- Reduce respiratory distress syndrome by ~50%.
- Reduce intraventricular haemorrhage.
- Reduce necrotising enterocolitis.
- Reduce neonatal death.
Two doses 24h apart; effect from ~24h after second dose.
Different scenarios — cervical length
Scenario 1: 20 weeks, cervix 35 mm at anomaly scan
Normal. No action. Routine care.
Scenario 2: 20 weeks, cervix 18 mm, first pregnancy
Short. Vaginal progesterone 200 mg nightly to 34-36 weeks. Serial cervical length scans. Watch for preterm labour signs.
Scenario 3: Previous 28-week delivery, this pregnancy 16 weeks
High risk. Consider history-indicated cerclage 12-14 wk. Serial cervical length scans. Progesterone. Specialist preterm clinic.
Scenario 4: 22 weeks, cervix 8 mm, bag visible
Rescue cerclage. Hospital admission. Progesterone. Steroids 24-28 wk. Monitoring intensive. Honest discussion about outcomes.
Scenario 5: Twin pregnancy + short cervix
Twins higher baseline preterm risk. Progesterone evidence in twins less strong than singletons. Cerclage decisions individualised. Specialist multi-fetal medicine team.
Care guidance — short cervix
- Routine anomaly scan often includes cervical length.
- High-risk women: serial scans from 14-16 wk.
- Progesterone nightly to 34-36 wk if short.
- Cerclage in selected cases.
- Avoid bed rest — not effective; may harm.
- Sensible activity; sex usually fine.
- Watch for preterm labour signs.
- Steroids 24-34 wk if birth threatening.
- Mental health support — anxiety common.
- Tommy’s resources for preterm birth support.
- Next pregnancy planning if recurrent risk.
Sources
- SMFM Consult Series #70 (2023). Cervical insufficiency and cervical cerclage.
- NICE NG25. Preterm labour and birth.
- Romero R, et al. Vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singletons with short cervix: meta-analysis. UOG 2018.
- RCOG Green-top Guideline 60. Cervical cerclage.
- Tommy’s. Preterm birth support.
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Frequently asked questions
What is a cervical length scan?
TRANSVAGINAL ULTRASOUND that measures how long your cervix is — the closed canal between the womb and the vagina. Done between 16 and 24 weeks of pregnancy (often at the 18-22 week anomaly scan). NORMAL length at this stage: >30 mm. SHORT (≤25 mm) is the single most powerful predictor of premature labour. NOT painful, takes a few minutes; smaller scanning probe than usual; you'll empty your bladder first.
When is cervical length scanning offered?
(1) UNIVERSAL screen at 18-22 weeks (anomaly scan) in some UK / US units — SMFM Consult #70 (2023) supports universal CL screening for singleton pregnancies; (2) HIGH-RISK pregnancies routinely scanned: previous preterm birth, previous cervical surgery (LLETZ/cone biopsy), multiple pregnancy (twins/triplets), uterine anomalies. NHS practice varies — some trusts universal, others targeted; PRIVATE scans available £150-300 for cervical length only or as part of anomaly scan package.
What does a short cervix mean?
≤25 mm at 18-24 WEEKS — increased risk of SPONTANEOUS PRETERM BIRTH (before 37 weeks). RISK varies with length: 25 mm → ~10% chance preterm; 15 mm → ~30%; 10 mm → ~50%; 5 mm → ~75%. NOT GUARANTEED — many short cervix pregnancies reach term with intervention. SHORT cervix in current pregnancy: 30-50% recurrence in next pregnancy. NOT YOUR FAULT — biological / anatomical / inflammatory factors; sometimes infection.
What if my cervix is short?
Several INTERVENTIONS may help: (1) VAGINAL PROGESTERONE (200 mg pessary nightly) from 16-22 to 34-36 weeks — REDUCES preterm birth by ~35% in short cervix pregnancies (Romero meta-analysis); (2) CERVICAL CERCLAGE (stitch) — surgical option in selected cases, especially if previous preterm + short cervix; (3) ARABIN PESSARY — silicone ring placed around cervix; mixed evidence; (4) RESTRICTED ACTIVITY — historically prescribed; modern evidence weak; (5) BED REST — no longer recommended; potentially harmful. SPECIALIST review needed.
What's a cervical stitch (cerclage)?
Surgical procedure where a STITCH is placed around the cervix to keep it closed. TYPES: (1) HISTORY-INDICATED — placed at 12-14 weeks based on previous late miscarriages / preterm births; (2) ULTRASOUND-INDICATED — placed at 16-22 weeks if short cervix on scan + previous preterm; (3) RESCUE / EMERGENCY — placed if cervix already dilated and bag visible. ANAESTHETIC: usually spinal/regional. STITCH REMOVED at 36-37 weeks (or in labour). NICE NG25 supports in selected cases. RISKS: infection, rupture of membranes, preterm labour from procedure (~1-5%).
Will the cervical length be checked again?
OFTEN YES — if first scan SHORT or HIGH-RISK. Typical schedule: every 1-2 weeks from 14-16 to 24 weeks. AFTER 24 WEEKS: less useful (cervix biology different). SOME PROGRESS / IMPROVE with progesterone; some shorten further; if marked shortening + previous preterm, cerclage discussed. NORMAL LENGTH stays stable / progressively shortens slightly near term (normal). HOSPITAL CARE PROTOCOLS vary.
Do I need to stop work / activity if cervix is short?
MODERN evidence: BED REST and STRICT activity restriction NOT recommended — may harm (DVT risk, muscle deconditioning, depression). SENSIBLE: avoid heavy lifting; consider reduced hours if physical job; sex generally fine (some restrict if very short cervix or bleeding); stress reduction. FIT NOTE possible if physical job. DISCUSS specifics with specialist. EVERY CASE DIFFERENT — clinical judgment + your preferences matter.
Is a short cervix my fault?
NO. Causes: (1) GENETIC / anatomical predisposition; (2) PREVIOUS CERVICAL SURGERY (LLETZ for abnormal smear, cone biopsy); (3) MULTIPLE PREGNANCY (uterine stretch); (4) UTERINE ANOMALIES (septate, bicornuate); (5) PRIOR PRETERM BIRTH (recurrent); (6) INFECTION sometimes; (7) IDIOPATHIC (no known cause). NOT caused by: exercise, sex, stress, foods, working. NOTHING YOU DID. Discuss with team; sometimes underlying treatable cause found.
Will I deliver early if my cervix is short?
NOT NECESSARILY. WITH TREATMENT: most short cervix singleton pregnancies reach term or near-term. CERCLAGE + PROGESTERONE combined often very effective. STATISTICS: short cervix at 22 wk without treatment: ~30-50% deliver before 35 weeks. WITH PROGESTERONE: ~20% deliver before 35 weeks (reduction ~35-40%). CERCLAGE: further reduces in selected cases. EACH WEEK GAINED reduces complications. EVEN IF preterm: 32-34 weeks now has excellent outcomes; 28-32 weeks good outcomes; <28 weeks more challenging but neonatal care advances rapidly.
Should my partner / family know I'm at higher risk?
USEFUL to share for support: (1) PRACTICAL HELP (childcare for older kids, food, household tasks); (2) EMOTIONAL support; (3) READY for early labour scenario; (4) PARTNER supportive at appointments. ANXIETY can be high; pre-term birth threat distressing. PEER SUPPORT: Tommy's UK has resources; preterm birth support groups online. MENTAL HEALTH: GAD-7 / PHQ-9 if anxiety affecting daily function.
What signs should I watch for?
PRETERM LABOUR signs (any gestation 22-37 weeks): (1) REGULAR contractions (every 10 min or more) — even if painless; (2) VAGINAL bleeding; (3) WATERS BREAKING (gush or trickle); (4) PRESSURE / heaviness in pelvis or backache; (5) UNUSUAL discharge (mucus plug); (6) REDUCED fetal movements. ANY OF THESE: GO TO MATERNITY UNIT same day. EARLY treatment (tocolytic + steroids) can delay birth 24-48 hours — gives steroids time to mature baby's lungs.
What about steroids before preterm birth?
ANTENATAL STEROIDS (BETAMETHASONE or DEXAMETHASONE) given between 24-34 weeks if preterm birth threatening: REDUCE: respiratory distress syndrome by ~50%; intraventricular haemorrhage; necrotising enterocolitis; neonatal death. TWO doses 24 hours apart; effect from ~24 hours after second dose. NICE / RCOG / ACOG recommend if delivery anticipated within 7 days. SAFE; minimal maternal side effects. RESCUE STEROIDS sometimes used if delivery now anticipated and weeks after first course.
What's a quipp app / fetal fibronectin?
QUIPP APP: tool combining QUantitative fetal fibronectin (fFN) + Cervical Length + history to predict preterm birth risk. fFN: protein in vaginal fluid; presence after 22 weeks suggests preterm labour risk. SWAB TEST. QUIPP: free app for clinicians; gives % risk of delivery within 14 days. USED in NHS to triage symptomatic women + asymptomatic high-risk women. /calculators/quipp-app for details. /calculators/fetal-fibronectin for fFN testing.
Can I have a vaginal birth with cervical stitch?
Stitch REMOVED at 36-37 weeks. AFTER removal: vaginal birth possible, normal labour can proceed. SOME WOMEN go into labour SOON after removal (cervix was held closed against pressure). SOME wait days-weeks. FEW women labour through stitch — emergency situation if happens. MOST stitched pregnancies reach term and have vaginal birth.
What about future pregnancies?
PREVIOUS SHORT cervix / preterm birth: ~30-50% recurrence next pregnancy. PRECONCEPTION discussion with consultant valuable. NEXT PREGNANCY plan: serial cervical length scans from 14 weeks; consider prophylactic progesterone; consider history-indicated cerclage at 12-14 weeks if strong history. NOT a sentence to repeat — many women have completely uneventful next pregnancies with monitoring + treatment.
How does this relate to other calculators on BumpBites?
Companion: /calculators/fetal-fibronectin for fFN testing; /calculators/quipp-app for risk calculation; /calculators/antenatal-steroids for timing; /calculators/vaginal-progesterone-ptb for progesterone in preterm prevention; /calculators/pregnancy-week for gestation; /calculators/contraction-timer for early labour; /calculators/birth-plan-builder.