Key Facts

Preterm labour is defined as the onset of regular painful uterine contractions leading to progressive cervical dilatation between 24+0 and 36+6 weeks of gestation. Preterm birth remains the leading cause of neonatal mortality and long-term disability.

Epidemiology

• 7-8% of births in the UK are preterm
• Approximately 60,000 preterm births per year in the UK
• Spontaneous preterm labour accounts for 70-80% of preterm births
• Iatrogenic preterm delivery (for maternal/fetal indication) accounts for 20-30%
• Incidence has been relatively stable or slightly increasing due to assisted reproduction

Aetiology

• Infection/inflammation (most common identifiable cause): chorioamnionitis, bacterial vaginosis, UTI
• Cervical insufficiency
• Uterine abnormalities (bicornuate uterus, fibroids)
• Multiple pregnancy
• Polyhydramnios
• Previous preterm birth (strongest risk factor: recurrence risk 15-50%)
• PPROM
• Placental abruption
• Social factors: smoking, low socioeconomic status, stress, domestic violence

Pathophysiology

• Multiple pathways converge on activation of the myometrium and cervical ripening
• Infection triggers inflammatory cascade (IL-1, IL-6, TNF-α) → prostaglandin production → uterine contractions
• Premature activation of the hypothalamic-pituitary-adrenal axis
• Cervical shortening and funnelling precede clinical labour
• Decidual haemorrhage/abruption activates thrombin which stimulates contractions

Symptoms

• Regular, painful uterine contractions (≥4 in 20 minutes)
• Lower abdominal cramping or pelvic pressure
• Low back pain (constant or rhythmic)
• Vaginal discharge (mucoid, bloody show)
• Sensation of pelvic pressure or heaviness

Signs

• Palpable regular uterine contractions on tocometry
• Cervical dilatation and/or effacement on examination
• Short cervical length on transvaginal ultrasound (<25mm is significant)

Red Flags

• Cervical dilatation >3cm (advanced preterm labour - tocolysis unlikely to be effective)
• Signs of chorioamnionitis (fever, tachycardia, uterine tenderness, offensive discharge)
• Vaginal bleeding (abruption)
• Abnormal CTG (fetal compromise)
• PPROM (pooling of liquor on speculum)

Bedside

• CTG (fetal monitoring)
• Speculum examination (look for cervical dilatation, pooling of liquor, bleeding)
• Fetal fibronectin (fFN) swab (if cervix <3cm dilated): negative result has >95% NPV for delivery within 7 days
• Urinalysis and urine MC&S
• Vital signs including temperature

Bloods

• FBC, CRP (infection screen)
• Blood cultures if pyrexial
• Group and save
• U&Es

Imaging

• Transvaginal cervical length measurement: <25mm is significant, <15mm is high risk
• Ultrasound for fetal presentation, weight estimation, liquor volume

Special Tests

• High vaginal swab and low vaginal swab for Group B Streptococcus
• Consider amniocentesis if infection suspected (rarely performed in UK practice)

Non-pharmacological

• Admit to unit with appropriate neonatal facilities
• In utero transfer to tertiary unit if <27 weeks gestation
• Neonatal team counselling regarding prognosis at given gestation
• Bed rest (limited evidence but commonly advised)

Pharmacological

• Antenatal corticosteroids: betamethasone 12mg IM × 2 doses (24 hours apart) if 24-34+6 weeks (NICE NG25). Single rescue course may be considered.
• Tocolysis: nifedipine 20mg PO stat then 10-20mg TDS-QDS (first-line, NICE NG25). Used to gain 48 hours for steroid effect and in utero transfer.
• Alternative tocolytic: atosiban (oxytocin receptor antagonist) 6.75mg IV bolus then infusion
• Magnesium sulphate for neuroprotection: 4g IV loading dose over 15 minutes if delivery expected within 24 hours at <30 weeks (reduces risk of cerebral palsy by 30%)
• Antibiotics: if evidence of infection (IV benzylpenicillin 3g then 1.5g 4-hourly for GBS prophylaxis)
• Erythromycin 250mg QDS PO for 10 days if PPROM (ORACLE trial)

Surgical/Interventional

• Cervical cerclage: rescue cerclage if cervical dilatation 1-4cm before 24 weeks with no infection (limited evidence)
• Prophylactic cerclage at 12-14 weeks for women with history of ≥3 preterm births or second-trimester losses
• Arabin pessary: alternative to cerclage (evidence debated)

Referral Criteria

• All women in suspected preterm labour require hospital assessment
• In utero transfer to tertiary unit if <27 weeks or anticipated neonatal surgical needs
• Preterm birth prevention clinic referral for women with risk factors in subsequent pregnancies

• Survival by gestational age: 24 weeks ~50%, 26 weeks ~75%, 28 weeks ~90%, 32 weeks ~95%, 34 weeks ~98%
• Major morbidity (cerebral palsy, chronic lung disease, severe disability): 24 weeks ~30%, 28 weeks ~10%, 32 weeks ~3%
• Antenatal corticosteroids reduce RDS by 34%, IVH by 46%, and neonatal death by 31%
• Magnesium sulphate reduces cerebral palsy by 30% when given before 30 weeks
• Recurrence risk of preterm birth: 15-50% (strongest predictor of subsequent preterm birth)

Survival and Outcome by Gestational Age

Key Trials